
J 2 y 




£7- 



Reprint, with additions, May 27, 1911. 

U. S. DEPARTMENT OF AGRICULTURE. 
BUREAU OF PLANT INDUSTRY— BULLETIN NO. 165. 

B. T. GALLOWAY, Chief of Bureau. 



APPLICATION OF SOME OF THE PRINCIPLES 
OF HEREDITY TO PLANT BREEDING. 



BY 

W. J. SPILLiVIAN, 

Agkicultukist m Chakge of the Office 
OF Farm Management. 



Issued December 31, 1909. 




WASHINGTON: " 
GO"\rEBNMENT PRINTING OFFICE. 
1911. 



Monograph 



i 

i 

i 



/ 



Reprint, with additions, May 27, 1911. 

U. S. DEPARTMENT OF AGRICULTURE. 
' ' BUREAU OF PLANT INDUSTRY— BULLETIN NO. 165. 

B.T.GALLOWAY Chief of Bureau. 



APPLICATION OF SOME OF THE PRINCIPLES 
OF HEREDITY TO PLANT BREEDING. 

BY 

W. J. SPILLMAN, 

Agriculturist in Charge of the Office 
OF Farm Management. 



Issued December 31, 1909. 




WASHINGTON: 
GOVERNMENT PRINTING OFFICE. 
1911. 



BUREAU OF PLANT INDUSTRY. 



Chief of Bureau, Beverly T. Galloway. 
Assistant Chief of Bureau, Albert F. Woods. 
Editor, J. E. Rockwell. 
Chief Clerk, James E. Jones. 



Office of Farm Management, 
scientific staff. 
William J. Spillman, Agriculturist in Charge. 
D. A. Brodie, David Griffiths, and C. B. Smith, Agriculturists. 

J. H. Arnold, Levi Chubbuck, M. E. McCulloch, A. D. McNair, G. E. Monroe, Harry Thompson, and 
E. H. Thomson, Experts. 

J. C. Beavers, G. A. Billings, M. C. Burritt, J. S. Gates, J. S. Cotton, H. R. Cox, M. A. Crosby, D. H. Doane, 
L. G. Dodge, J. A. Drake, L. W. Ellis, J. W. Froley, C. L. Goodrich, Byron Hunter, H. B. McClure, 
J. C. McDowell, H. A. Miller, W. A. Peck, J. A. Warren, and B. Youngblood, Assistant Agriculturists. 
C. M. Bennett, M. O. Bugby, E. L. Hayes, M. M. OfEutt, A. G. Smith, E. A. Stanford, and G. J. Street 
Special Agents. 
165 
2 



PC' 



LETl^ER OF TRANSMITTAL 



U. S. Department of Agriculture, 

Bureau of Plant Industry, 

Office of the Chief, 
Washington, D. C, August 28, 1909. 

Sir: I have the honor to transmit herewith and to recommend for 
pubhcation as Bulletin No. 165 of the series of this Bureau the accom- 
panying manuscript entitled ''Application of Some of the Principles of 
Heredity to Plant Breeding." This paper was prepared by Mr. W. J. 
Spillman, Agriculturist in Charge of the Office of Farm Manage- 
ment of this Bureau. Great progress has been made during the past 
ten years in investigations relating to the principles involved in the 
improvement of plant varieties and the production of new varieties 
by cross-breeding. Thus far there has been no general statement 
of the principles applicable in this work especially designed for the use 
of the actual breeder. The present paper is an attempt to set forth 
in an orderly manner what is known of the effect of selection on differ- 
ent types of plants and the possibilities of cross-breeding for the pur-- 
pose of producing new varieties, as understood by the author. 

The paper is submitted and recommended for publication in accord- 
ance with the fixed policy of this Bureau of giving its men full oppor- 
tunity of presenting results of scientific and practical interest from 
different points of view. 

The author wishes to acknowledge the helpful criticism of Prof. 
C. V. Piper, and especiall}^ of Assistant Secretary Willet M. Hays, 
both of whom have carefully read the manuscript and have made 
many valuable suggestions in the treatment of the various topics. 
Respectfully, 

B. T. Galloway, 

Chief of Bureau. 

Hon. James Wilson, 

Secretary of Agriculture. 

165 3 



CONTENTS. 



Page. 

Introduction 7 

Dominance and recessiveness 7 

Segregation 8 

Allelomorphism 17 

Law of recombination 18 

Fluctuating variations 23 

Running out of varieties 25 

Selection without artificial crossing 26 

Vegetative propagation 26 

Self-fertilized species 32 

Mass selection 33 

Individual selection 35 

Cross-fertilized species 36 

Hybridization and selection 46 

Vegetatively propagated crops 46 

Self -fertilized species 47 

Cross-fertilized species 53 

Mendelian analysis of heterozygote races 54 

Heterozygote characters 55 

Possibility of entirely new characters 58 

Reciprocal crosses 59 

Evolutionary changes and their relation to plant breeding 59 

Place effect 61 

Non-Mendelian characters 62 

Mutations 64 

Latency 65 

I. Latency due to separation 65 

II. Latency due to dominance of absence over presence 67 

III. Latency due to homozygosis 68 

IV. Latency due to hypostasis (masking) 69 

V. Latency due to inhibition 69 

VI. Latency due to fluctuation : 69 

Correlation 70 

Index 71 

165 5 



ILLUSTRATIONS. 



Page. 



Fig. 1. Graphic illustration of the range of fluctuations of each of the eight pure 

races of Paramecium studied by Jennings 28 

2. Graphic illustration of ten generations of corn with no selection, the 

first generation of which is YySs 38 

3. Graphic illustration of the effect of mass selection in cross-fertilized 

species 42 

4. Graphic illustration of ten generations of a hybrid in a self -fertilized 

species without selection to type 50 

5. Graphic illustration of ten generations of a hybrid in a self-fertilized 

species selected for type WWCC 52 

6. Graphic illustration of the effect of individual selection in a self- 

fertilized species on progeny of the hybrid WwCc 53 

165 
6 



B. P. I.— 513. 



APPLICATION OF SOME OF THE PRINCIPLES 
OF HEREDITY TO PLANT BREEDING. 



INTRODUCTION. 

While the discussion in these pages of principles that may be applied 
in the improvement of crops by breeding and selection will involve 
principles other than those discovered by Gregor Mendel, the fact 
that Mendel's principles are somewhat complex renders it necessary 
to state them in a general way before taking up the subject of plant 
improvement. 

DOMINANCE AND RECESSIVENESS. 

The simplest of the principles discovered by Mendel is that which 
is usually referred to as the ^^aw of dominance." This principle 
should hardly be called a law, because it is in no wise general and in 
very few cases is dominance absolute. The phenomena of dominance 
and recessiveness may be illustrated by a few examples. 

If a red-flowered variety of the common garden pea be crossed 
with a white-flowered variety, the progeny will have red flowers. 
According to Mendel's original conception a cross of this kind brings 
together two antagonistic characters. The progeny inherit the red 
flower color from one parent and the white flower color from the 
other. It therefore has both these characters. It happens, however, 
that the red character predominates over the white and comes to 
expression while the white character is not visible in the cross-bred 
individual. Mendel suggested that a character behaving as the red 
character does in this cross should be called a '^dominant character," 
while one behaving as the white character in this cross should be 
called a ^'recessive character." 

If we cross a bearded variety of wheat with a smooth variety, that 
is, one that has no beards, the hybrids thus produced either have no 
beards or the beards will be only slightly developed. Hence, we say 
that smoothness is dominant to beards, at least partially, or, which 
means the same thing, that beards are recessive to smoothness. The 
cross between polled and horned breeds of cattle has no horns, though 
a small proportion of such cross-bred animals may have ^'scurs," 

165 7 



8 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 

that is, imperfect horns. Hence, we say that horns are recessive 
and the poll character dominant. Many other cases might be cited 
to illustrate dominance and recessiveness of hereditary characters, 
but the above examples will serve to illustrate the principles suffi- 
ciently here. 

While it is not uncommon for a character to be dominant or reces- 
sive in a cross, it is seldom that dominance is absolute. The presence 
of the recessive character can usually be detected, and in some cases 
very easily. Thus, in the cross between bearded and smooth wheat 
the hybrids usually show a slight tendency to be bearded. Likewise, 
as already stated, the cross between horned and polled cattle may 
have scurs. It frequently happens that instead of either of two 
opposite characters being dominant we get a form intermediate be- 
tween the two parent forms. Thus, in the cross between ordinary 
long-headed wheat and the short-headed club wheats of the Pacific 
coast the hybrid has heads of intermediate length, though they are 
much more like club wheat than they are like the ordinary kinds, so 
that the club character is at least partially dominant. In certain 
crosses between red-flowered and white-flowered ornamental plants 
the hybrids are pink. 

In not a few instances a hybrid is altogether different in some 
characters from either of its parents. Thus, in the case of the cross 
between a certain red primrose and a certain nearly related white 
variety the hybrid is purple. 

We thus have every gradation between perfect dominance of a 
character over its opposite and cases in which the hybrid is unlike 
either parent. 

SEGREGATION. 

We have seen that when two naturally opposite characters meet 
in the same individual one of them may be completely dominant, 
as the poll character in many individuals of the cross between polled 
and horned cattle, or the crossbred individual may exhibit a char- 
acter intermediate between the opposed characters of its parents, 
as the pink color of certain hybrids between red-flowered and white- 
flowered plant varieties, or the hybrid may exhibit a character 
different from the corresponding characters of either of its parents, 
as the purple color of hybrid primroses produced by crossing cer- 
tain red and white varieties. 

In a pure race of plants having red flowers we may assume that 
each individual which bears seed transmits to all its seed the tend- 
ency to produce red flowers. Likewise, in a pure wliite-flowered 
race, each individual transmits to its progeny the tendency to pro- 
duce white flowers. But what of the hybrid between two such 

165 



SEGREGATION. 



9 



races ? What does this hybrid transmit to its offspring ? Let us con- 
sider the case of the hybrid primrose having purple flowers. The 
facts are, as found by experiment, that this purple hybrid produces 
three kinds of progeny. About one-fourth of the seed produced by 
this hybrid produces plants having red flowers like those of the red- 
flowered parent of the hybrid. Another fourth have white flowers, 
while the remaining half have purple flowers. Furthermore, the 
red and the white flowered plants of this second generation will repro- 
duce only red or white progeny, as the case may be; that is, they 
behave exactly like pure red or pure white races. On the other 
hand, every one of the purple-flowered plants will produce in the 
next generation three kinds of progeny as before. One-fourth of 
the progeny of these purple-flowered plants will have red flowers, 
one-fourth of them white flowers, and half of them purple flowers. 
This experiment was continued by an English florist for fifteen 
years, always with the same result. The purple always split up 
into one-fourth red, one-fourth white, and one-half purple, while 
the reds and the whites thus produced always behaved like pure 
races of red or white. From these facts we infer that in self -fertilized 
species an individual which is hybrid with reference to a particular 
pair of characters tends to produce progeny one-fourth of which 
is of pure race like one of the parents of the hybrid, another fourth of 
pure race like the other parent, while the remaining half is hybrid 
like the original hybrid itself. 

Mendel suggested that the cause of these peculiar phenomena is 
that the hybrid produces two kinds of ovules and two kinds of pollen, 
the one kind of ovule and one kind of pollen being exactly like those 
of one of the parents of the hybrid so far as the one character under 
consideration is concerned, the other kind being like those of the 
other parent. Let us see how this hypothesis fits the facts. 

Suppose the hybrid does produce two kinds of ovules in equal 
numbers, one of which carries the potentiality of the red flower 
color, the other that of the white, and two kinds of pollen differing 
in a similar manner. Let us designate the ovules and pollen car- 
rying red by the letter R, and those carrying white by W. Let us 
first consider what happens to the ovules of type R. These ovules 
are offered both R and TF pollen in equal quantities. The chances 
are, therefore, that half the R ovifles wiU be fertilized by R pollen 
and the other half by TF pollen. In the first of these cases, we have 
R ovules fertilized by R pollen, Avhich would, of course, give pure 
red individuals. For convenience, we may designate these individ- 
uals resulting from the fertilization of R ovules by R pollen as RR 
individuals. Since half of the ovules produced by the hybrid are 
supposed to be of type R and since half of these are fertilized by R 

165 



10 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 

pollen it follows that one-fourth of the progeny of the hybrid will be 
pure reds. Similarly, the W ovules are offered both kinds of pollen, 
and the chances are that half of these ovules, or about half, will be 
fertilized by R pollen, the other half by W pollen. The latter half 
being fertilized by pollen of their own kind result in pure white 
individuals, which we may, for convenience, designate as WW indi- 
viduals, thus indicating that both the ovules and the pollen which 
gave rise to these individuals had the character W. The WW indi- 
viduals also constitute one-fourth of the progeny of the hybrid. 
The remaining half of the progeny result from the fertilization of 
one kind of ovule by the opposite kind of pollen, thus giving hybrids 
like the original hybrid, which we may designate by the formula R W. 

We thus see that the supposition that the hybrid produces two 
kinds of pollen, one like the pollen of the red variety and the other 
like that of the white, and tw^o kinds of ovules, differing in a similar 
manner, fully explains the phenomena observed by the breeder of 
the purple primrose. This hypothesis is further substantiated by 
the following facts. If we apply the pollen of the hybrid to the 
stigmas of the red variety, half the progeny thus obtained will be 
red and half purple. This is easily understood if the hybrid pro- 
duces two kinds of pollen in equal quantities. All the ovules of the 
red variety have the character R. If half the pollen of the hybrid 
carries R, then half the progeny will be RR, or pure red. If the 
other half of the pollen carries W, then the other half of the progeny 
will be of the type R W. That the hybrid produces two kinds of 
ovules is shown also by the fact that if we apply pollen of the red 
variety to the stigmas of the hybrid, half the resulting progeny will 
be red and half purple. 

We may accept the hypothesis, therefore, that a hybrid plant 
whose parents differ in respect to a single character pair produces 
two kinds of ovules and two kinds of pollen, one kind being like 
those of one of its parents, the other kind like those of its other 
parent. 

If a hybrid which has in its cells two characters which are natur- 
ally the opposite of each other can not produce ovules and pollen 
with both of these characters in the same ovule or pollen grain, then 
it follows that these two opposite characters can not be transmitted 
together. They remain together in the cells of the hybrid well 
enough, but they fall apart somewhere in the process of producing 
reproductive cells. Let us now inquire how this segregation of the 
members of a pair of opposite characters into different ovules and 
different pollen grains, which takes place in hybrids, may occur. 
In the cells of a plant we have, first, the outer covering, or cell wall. 
Within is the nucleus, between which and the cell wall hes the 

165 



SEGBEGATION. 



11 



cytoplasm, consisting of a semi-liquid ground substance, in which 
lies the network of the cytoplasmic reticulum, in the meshes of 
which occur various small bodies called collectively the cytoplasts. 
Within the nucleus, which is separated from the cytoplasm by the 
nuclear membrane, are found the chromosomes, which are small 
bodies of living substance lying in the nuclear sap or ground sub- 
stance of the nucleus. We must seek for the potentialities of the 
hereditary characters either in some of these cell organs or in their 
relations to each other. The behavior of the chromosomes is such 
as to suggest strongly that they are the seat of at least some of the 
potentialities in question. The work of Prof. E. B. Wilson and his 
pupils and others indicates that in certain animals certain identifiable 
chromosomes are responsible for the differences between the sexes, 
at least for the primary sexual differences. Several other hereditary 
characters not directly related to sex behave in such manner as to 
indicate clearly that they bear to the chromosomes a relation similar 
to that which sex bears to these cell organs. It is highly probable, 
therefore, that many hereditary characters depend in some way not 
yet understood on the chromosomes. In fact, when we describe 
the known behavior of the chromosomes we describe the known 
behavior of Mendelian characters. 

It is not necessary in this discussion to consider the various theo- 
ries regarding the relation of hereditary characters to the organs 
of the cell. The behavior of the characters studied by Mendel and 
of hundreds of characters investigated by others leaves no doubt 
that these characters depend in some way on definite cell organs. 
This does not necessarily imply that each hereditary peculiarity 
of a race is represented by a distinct body in the germ cells. A given 
peculiarity may be due to peculiarities in the composition or the 
physiological behavior of several cell organs. This much, however, 
seems to be certain: When two races differ in respect of a character 
and when the hybrid between these races produces two kinds of 
reproductive cells, one of which is like the reproductive cells of one 
of its parents and the other like those of the other parent, as is the 
case in primroses just cited, then the difference between these 
two races is due to differences iit a single cell organ or to a group 
of such organs which act together at all times as if they were insep- 
arable. In hybridization we are dealing with differences between 
organisms, and these differences are due to differences between 
corresponding cell organs in the different races. For instance, sup- 
pose we have two races of plants which differ only in the fact that one 
of them has red flowers and the other white and that the hybrid 
between them produces two kinds of pollen, one of which is identical 
with the pollen of the red variety and the other with that of the 

165 



12 APPLICATION OF PRINCIPLES OF HEEEDITY TO BREEDING. 

white variety. We know that the real difference between these 
varieties hes in the fact that one of them produces red coloring mat- 
ter and the other does not. We may therefore assume that in the 
white variety a certain cell organ fails to perform a function which 
the corresponding organ in the other variety does perform. We 
may call this ' function which is performed in the red variety the 
'^determiner" for red. In the white variety this determiner is 
absent, although the cell organ which performs this function in the 
other variety may be present in the white variety. In this variety 
it fails to perform the function necessary to the production of red 
coloring matter. 

We should not get the idea that red coloring matter is due wholly 
to a single function of a single body, for such is probably not the case. 
It may be necessary for several cell bodies to cooperate in the pro- 
duction of this substance. In the white-flowered variety all of these 
bodies may function properly except one, the failure of the one 
body to perform its appropriate function being responsible for the 
nonproduction of the red coloring matter. But when we are deal- 
ing with a cross between these two varieties it is the one point in 
which they difl^er that concerns us, and we shall use the word deter- 
miner" to apply to this point of difference. Hence, we say that in 
the one variety the determiner for red is present and in the other 
it is absent. 

Although the determiner of a character is assumed to be a function 
of a definite body, or of several such, we shall not attempt in what 
follows to distinguish in all cases between these bodies and their 
functions. In general, we shall represent the determiner for a 
character by a capital letter, usually the initial letter of the name of 
the character. Thus, capital R may be taken as the symbol of 
the determiner for red coloring matter, but this symbol will be used 
indifferently for the function which produces red and for the body 
or group of bodies which has this function. For the absence of this 
determiner in the white variety we shall use the corresponding 
small letter. Thus, r may be considered in what follows as repre- 
senting the absence of the function R, or it may be considered to rep- 
resent the body present in the white variety that fails to perform the 
function which is performed by the corresponding body in the red 
variety. 

We are now ready to explain wh}^ the hybrid between a red and 
a white variety of primrose produces two kinds of reproductive cells, 
one like those of the red variety and one like those of the white — at 
least, to offer an hypothesis that agrees with the facts. 

The red variety has inherited the determiner for red from two 
parents. The condition of this determiner in the red variety may 

165 



SEGKEGATION". 



13 



therefore be represented by the symbol RR. This means that in 
the cells of a plant of the red variety there are two determiners for 
red. The corresponding determiners in the white variety may be 
represented by the symbol rr, wliicli may be taken as representing 
two bodies, neither of wliich performs the function necessary to the 
production of red coloring matter, but wliich correspond in the white 
variety to homologous bodies wliich do perform this function in the 
red variet}^ The sjanbol of the hybrid would, of course, be Rr, in 
which R represents the active" determiner derived from the red 
variety and r the nonactive one from the white variety. In ordinary 
growth, when a body cell has attained its maturity and divides into 
two cells it is supposed that each character determiner present 
divides, one part going into one of the new cells, the other into the 
other. Thus, if a mature cell contains the determiners R and r, 
then each of the new cells formed by its division likewise contains 
both R and r. Thus every cell in the body of the individual may be 
supposed to have both R and r in it. This is certainly true of those 
cells which form the direct hne of descent from the original fertihzed 
ovule to the new ovules and pollen grains produced by the individual. 
The cells in this line of descent are called collectively the germ cells, 
a term which we shall find convenient to use. 

If ovules and pollen cells were formed by ordinary cell division 
such as that described above, it is clear that every ovule and every 
pollen grain produced by the hybrid Rr would contain both R and r. 
But the facts indicate that only half the ovules and half the pollen 
grains contain R, while the other half contain r. There must be, 
then, a cell division somewhere in the line of descent which differs 
from the ordinary type of cell division, and there is unmistakable 
cytological evidence that such is the case. Just before the forma- 
tion of ovules and pollen grains (in fact, in next to the last division 
of the germ cells) we find a cell division in which the chromosomes 
do not divide in the usual manner. Instead they unite in pairs, 
forming double, or bivalent, chromosomes. This union of chromo- 
somes into pairs reduces the number of chromosomes to half what 
it w^as before. Then, when the cell divides, these large chromosomes 
divide, presumably into the two halves which united to form them. 
If we call the large double chromosomes mother chromosomes and 
the small ones into wliich they separate daughter chromosomes, 
then in this cell division one of the daughter chromosomes passes 
to one of the newly formed cells, wliile the other passes to the other 
cell. Now, if these chromosomes either themselves are the bodies 
whose functions are our ^^determiners," or if they contain smaller 
bodies which are responsible for the determiners, we have at once 
an explanation of the fact that our hybrid produces two kinds of 

165 



14 APPLICATION OF PRINCIPLES OF HEEEDITY TO BREEDING. 

ovules and two kinds of pollen. For in this cell division, which is 
called the reduction division because in it the number of chromo- 
somes is reduced by half, our determiners R and r may be assumed 
to reside in separate chromosomes which unite to form a single 
bivalent. While cell division is taking place this bivalent chromo- 
some again separates, R passing into one daughter cell and r into 
the other. If the determiners R and r are not simply the functions 
of chromosomes themselves they at least pertain to bodies which 
at some point in the hne of descent of the germ cells behave just as 
we know the chromosomes do behave — that is, at some cell division 
R and r unite into a pair, and when division occurs R goes one way 
and r the other. Two determiners which thus behave tow^ard each 
other are said to constitute a Mendehan pair. 

Most Mendelian pairs consist simply of the presence of a given 
determiner on the one hand and the absence of that determiner on 
the other. Furthermore, the determiner which represents the pres- 
ence of a character is in nearly all cases dominant over the deter- 
miner which represents the absence of that character. Dr. C. B. 
Davenport, of the station for experimental evolution of the Car- 
negie Institution, found that in poultry practically all the character 
pairs known show this relation; i. e., presence of a character domi- 
nant and absence of it recessive. We have seen, however, that there 
are some exceptions to this rule, since the poll character is dominant 
to horns and beardlessness in wheats is dominant to beards. The 
difference between polled and horned cattle is the absence of horns 
in one and their presence in the other. 

But cases are known in which this simple relation of presence and 
absence of a character does not constitute the Mendelian pair. For 
instance, if Barred Plymouth Rock females be mated with Indian 
Game males all the female progeny of this moating will be black, while 
all the males will be barred like the mother. Data accumulated by 
the writer and an extended series of experiments performed by Mr. 
H. D. Goodale " indicate that the female Barred Plymouth Rock pro- 
duces two kinds of eggs. One of these kinds is destined to produce 
females, and these female-producing eggs do not have the determiner 
for barring in them. The other kind is destined to produce males, and 
these eggs do have the barring factor. In this case it appears, there- 
fore, that the determiner for femaleness and that for barring form a 
Mendehan pair. Wilson has apparently shown that the determiner 
for femaleness in many animals is a certain chromosome or group of 
chromosomes that always act as a unit. If we assume that the deter- 
miner for barring is another chromosome which unites with the sex 
element to form a bivalent in the reduction division we have at once 
an explanation of the behavior of the determiner for barring. In the 



165 



a See Science, June 25, 1909. 



SEGREGATION. 



15 



germ cells of the female Barred Plymouth Rock we have the two ele- 
ments F, which causes the individual to be a female, and B, which gives 
the barring on the feathers. In ordinary cell division both of these 
elements divide; but in the reduction division F and B unite into 
one body. This body then separates as the cell divides, so that one 
of the daughter cells contains F and the other B. Such a female 
therefore produces two Idnds of eggs, one containing the determiner 
for the female sex, the other that for barred feathers. 

Several other cases are known in which Mendelian pairs are formed 
of determiners for characters that are apparently unrelated. Such 
a case occurs in the purple primrose previously mentioned. We may 
explain the phenomena presented by this hybrid as follows: Let us 
assume that the original wild species from which the red and the 
white races in question are descended had purple flowers and that 
this purple color was due to two functions of the same cell organ. 
One of these functions, which we may designate as R, had to do with 
the production of red coloring matter, while the other, which we may 
designate as P, changed red into purple, somewhat after the manner 
in which an alkali changes litmus from red to blue.'^ The determiners 
R and P were functions of the same cell organ, probably a chromo- 
some. Since this body had two functions we may represent it by 
the symbol R-P, the hyphen indicating that the two functions 
belong to the same cell organ. Since there is a pair of these bodies 
in each cell, the complete status of these determiners in the body 
cells would be represented by R-P R-P. 

Now, let us suppose that in one section of the species the deter- 
miner R becomes latent or is lost. Our formula then becomes 
r-P r-P, or simply PP in this race, which would, of course, have 
white flowers, since the determiner for red is absent. In another 
section of the species the determiner P vanishes, leaving R-p R-p 
or simply RR. Now, when we cross these two races we bring the 
determiner r-P into the same cell with R-p. Here we have the red 
coloring matter produced by one determiner and converted into 
purple by the other. This would account for the purple color of the 
hybrid, as well as the red of one variety and the white of the other. 
Other cases of Mendelian pairs of this nature will be mentioned 
later. 

In this purple hybrid we may consider that we have one character 
pair consisting of R from the red parent and r from the white parent, 
so that this pair consists of red and absence of red, while along 
with it we have another pair consisting of P from the white parent 
and p from the red parent, so that this pair consists of the presence 
of P and the absence of P. But the fact that P and R can not be 
transmitted together indicates that R and p pertain to the same 



«See article by Shull in American Naturalist, July, 1909, 
81599°— Bui. 165—11 2 



16 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 

cell organ, and that r and P pertain to the Mendelian mate of this 
organ. Since most Mendelian character pairs consist of determiners 
one of which represents the presence of something and the other 
the absence of the same thing, we shall, in general, use for such pairs 
of characters in hybrids symbols consisting of a capital letter and 
the corresponding small letter, the capital letter standing for presence 
of the character and the small letter for its absence. Thus, in the 
cross between red and white peas, since the difference between 
these varieties consists in the presence of red color in one and its 
absence in the other, we represent the hybrid as Rr. Since the 
hybrid itself is red in this instance, this formula is logical; it would 
naturally be red because of the presence of R. But, as previously 
stated, there are cases in which the absence of a character is domi- 
nant in the hybrid between races one of which has the character and 
the other does not. Thus the hybrid between polled and horned 
cattle is polled. Here the formula for the horn determiner in the 
pure horned breed would be HH; in the pure polled breed, M; and 
in the hybrid, Hli. But since the determiner H does not succeed 
in producing horns in the hybrid and the hybrid therefore has the 
appearance of its polled parent we may write the forniula for the 
hybrid as {H)lij to show these facts. Similarly, the hybrid between 
bearded and smooth wheat would be represented by 

Cases like the purple hybrid primrose are so rare that we do not 
need to use any particular symbol to indicate that the hybrid is unlike 
either parent. 

The question why these characters, horns in cattle and beards in 
wheat, do not develop when represented by only one active deter- 
miner is an interesting one, and is very ably discussed by Doctor 
Shull in the July, 1909, number of the American Naturalist. The 
fact probably is that in these hybrids the determiners H and B are 
not latent, but that single determiners are not able to produce that 
chemical condition in the cell which is necessary for the develop- 
ment of these characters. In pure horned cattle and pure bearded 
wheat, where there are two active determiners for each of these 
characters, the proper condition for the development of these char- 
acters is brought about. 

In general, a hybrid produces three types of progeny with ref- 
erence to each pair of characters in which its parents differ. The 
hybrid between red and white varieties of peas produces two kinds 
of pollen, which we may designate as R pollen and r pollen. It pro- 
duces two corresponding types of ovules. On the average, half the 
R ovules are fertilized by R pollen, so that one-fourth of the progeny 
of such a hybrid is of the type RR or pure red. Likewise, half the r 
ovules are fertilized by r pollen, giving rr individuals, which consti- 

165 



ALLELOMORPHISM. 



17 



tute one-fourth of the second generation. The remaining half of the 
R ovules meet r pollen and the remaining half of the r ovules meet 
B pollen, giving in each instance the combination Rr, which con- 
stitutes half the second generation. If R is completely dominant 
the types RR and Rr can not be distinguished, since the latter has 
red flowers like those of type RR. Hence, where dominance is com- 
plete the second generation appears to consist of only two types. 
One of these types shows the dominant character, the other the 
recessive character, and the dominant type is three times as numer- 
ous as the recessive. Thus we arrive at the well-known Mendelian 
ratio of 3:1, or three dominants to one recessive in the second gen- 
eration of a hybrid. 

In the above second generation the two types RR and rr are seen 
to consist of like things united, while the type Rr consists of unlike 
things united. Types RR and rr are said to be homozygote, a term 
which means ^^ike things united," while i?r is said to be heterozygote, 
which means ^ ^unlike things united." An individual is said to be 
homozygote with reference to a given character when the cells of that 
individual contain two determiners for the presence of that character. 
If its cells contain only one determiner for any character it is said to 
be heterozygote for that character. Thus a bearded w^heat is homo- 
zygote for beards, a pure race of smooth wheat is homoz3^gote for 
absence of beards, while a cross between a bearded and a smooth race 
is heterozygote for beards. 

ALLELOMORPHISM. 

The term ^^allelomorph" was introduced by Prof. William Bateson, 
of Cambridge, England, one of the leading investigators of Mendelian 
phenomena. It is derived from two Greek words, one of which 
means ^^one another" and the other ^^form." We may say that it 
means ''corresponding forms." What we have called a ''pair of 
determiners" Bateson calls a "pair of allelomorphs." The term " allelo- 
morph," however, has a wider application than "determiner;" it may 
mean characters themselves as well as the determiners of those char- 
acters. To say that one character is allelomorphic to another means 
simply that the two characters when brought together in the same 
individual form a Mendelian pair and hence fall apart when repro- 
ductive cells are produced. Thus, a pair of allelomorphs is what we 
have been calling a "pair of Mendelian characters." Hence, the 
term "allelomorph" is frequently used simply to mean a Mendelian 
character; that is, a character which obeys Mendel's law of segregating 
from its mate in the reduction division. 

The term "gamete" is also a very convenient one which we shall 
have occasion to use frequently. It simply means a reproductive cell, 
such as an ovule, a pollen grain, an unfertilized egg, etc. 

165 



18 APPLICATION OF PRINCIPLES OF HEHEDITY TO BREEDING. 



LAW OF RECOMBINATION. 

The third and most important principle discovered by Mendel" is 
the fact that, generally speaking, when two or more ''pairs" of char- 
acters are present in the same hybrid these pairs are independent of 
each other, so that one member of a given pair may be transmitted 
with either member of another pair. The results of this important 
discovery are shown in Table I, w^hich illustrates the cross between 
Polled Durham and Hereford cattle. 

As is well known. Polled Durham cattle have colored faces and no 
horns, while Herefords have horns and white faces. The white face 
of the Hereford seems to be due to the presence of a determiner which 
controls the distribution of color over the body. We thus represent 
white face by W and colored face by w, that is, absence of white face. 
As before, the poll character is represented by h and the horn char- 
acter by H. White face is dominant to colored face in this cross. 
The complete formulae for these two pairs of characters in the body 
cells are, therefore — 

In pure Hereford cattle, HHWW. 
In Polled Durham cattle, hhww. 
In the cross, {H)hWw. 

The cross has the white face but no visible horns, though it may 
have scurs. 

The squares in the upper part of Table I represent germ mother 
cells dividing in the reduction division. In this division each pair of 
characters is separated. Thus, the pair Hli, both members of which 
have been present in every cell of the body of the hybrid, is here 
separated, H going to one daughter cell and Ti to the other. In the 
cross here under consideration we have a second pair of allelomorphs, 
namely, Ww. When a given mother cell divides, the two pairs of 
allelomorphs may be arranged as in the left-hand square at the top 
of Table I, in wdiich case // and w go together into one daughter cell, 
while li and ^¥ go into the other. Such a division gives two kinds of 
gametes, the formulae for which are, respectively, IIw and liW} Or 
the two pairs of allelomorphs may be arranged as in the right-hand 
square of Table I, in which case H and W go to the same daughter 
cell, while li and w go to the other, giving two kinds of gametes having 
the respective formulae // W and Jiw. There are, in all, therefore, four 

a The first is the so-called "law of dominance," though it is hardly entitled to rank 
as a law; the second is the law of segregation of character pairs. 

&The gametic formula Hw does not represent a pair of determiners. It represents 
two determiners, one of which is from one pair and the other from another pair. We 
do not have pairs of determiners in gametes, i. e., in reproductive cells. The pairs 
separate in the reduction division, and a gamete never has both members of the same 
pair. 

165 



LAW OF RECOMBIKATION. 



19 



kinds of gametes that a hybrid individual of the type here under con- 
sideration can produce, namely, Hw, 7iW, HW, and Jiw. In the first 
of these four types of gametes, namely, Hw, we find horns being trans- 
mitted with colored face; in the third, HW, horns and white face are 
transmitted together. Likewise, in the second type, Ji W, we have the 
poll character and white face together, while in type four, 7iw, we have 
the poll character and colored face. 

Since in the reduction division either of the two possible arrange- 
ments of the two pairs of allelomorphs is just as likely to occur as the 
other, one of them will occur in about half the cells and the other in 
the other half. We thus get all four types of gametes in equal num- 
bers in every hybrid animal of this character. In the male this is 
actually realized, for millions of gametes are produced. But in the 
female only a few reproductive cells are formed, but these few are as 
likely to be of one type as another. Hence, on the average for a large 
number of such females, the four types of gametes will occur equally 
often. In the middle part of Table I we have all the possible, and 
equally probable, unions that can occur between the gametes of the 
two sexes. Thus the Hw gametes of the female are offered four kinds 
of sperm in equal numbers. Hence, on the average one-fourth of these 
Hw ovules will be fertilized by Hw sperm, giving individuals of the 
next generation of the type HHww. Another fourth will meet liW 
sperm, giving individuals of the type {H)liWw, and so on. 



Table I. — Cross between Polled Durham and Hereford cattle and its results. 



Reduction division as itj 
occurs in the cells of I 
the first-generation hy- j 
brid. J 
The four types of repro-i 
ductive cells produced 
by first-generation hy-[ 
brids. J 
Or simply Hw, hW, HW, hiv 



The sixteen possible, and equally probable, matings to produce second-generation 
hybrids are as follows: 

Gametes of male. 





Hw 


hW 


HW 


hw 


Hw 


1 HHww 


2 (H) hWw 


3 HHWw 


4 (H) hww 


hW 


5 (J?) hWw 


6 hhWW 


7 (H)hWW 


8 hhWw 


HW 


9 HHWw 


10 (H) hWW 


11 HHWW 


12 (H) hWw 


hw 


13 {H)hww 


14 hhWw 


15 (H) hWw 


16 hhww 



165 



20 APPLICATION OF PEINCIPLES OF HEREDITY TO BREEDINa. 



The nine different combinations and their relative frequency follow: 

1 HHWW. 2 {H)hWW. 1 hhWW. 

2 HHWw. 4 {H) hWw. 2 hhWw. 
1 HHww. 2 (77) hww. 1 hhww. 

The sixteen formulae in the squares in the middle of Table I show 
the results of these sixteen unions. It will be seen that some of 
these matings are alike; for instance, 2, 5, 12, and 15; 3 and 9; 4 
and 13. There are only nine different kinds, as shown in the lower 
part of Table I. These nine different combinations occur in the 
relative frequencies shown in the numbers preceding each of the 
nine in the lower part of Table I. Thus, one-sixteenth of the progeny 
will represent the combination HHWW, four-sixteenths the combi- 
nation (H)'h Ww, and so on. 

Table II shows the results of a more complex cross which the 
writer made while connected with the Washington Agricultural 
Experiment Station, Pullman, Wash. It is a cross between two 
varieties of wheat, one of which was a winter wheat that lodged 
easily (that is, had weak straw) and had open chaff, and thus when 
ripe shattered its grain easily. The other was a variety of spring 
wheat that did not lodge and had tightly closed chaff when ripe. 
The first generation ° of the hybrid inherited a very complex lot of 
characters. Thus, it inherited both winter and spring character; 
both the lodging and the nonlodging tendency; both the open and 
the closed chaff tendency. In this cross the winter character, the 
nonlodging tendency, and the closed-chaff tendency were dominant. 

Letting — 

TF stand for the winter character, 

w for absence of winter character (i. e., spring character), 

iVfor nonlodging (i. e., for stiff straw), 

n for absence of N (i. e., for weak straw), 

C for closed chaff, and 

c for absence of C (i. e., for open chaff), 

the formula of the hybrid was WwNnCc. Now this hybrid can 
produce, and does produce, in about equal numbers eight different 
types of ovules and eight similar types of pollen, namely, WNC, WNc, 
WnC, Wnc, wNC, wNc, wnC, wnc. The union of these eight types of 
ovules and pollen grains gives sixty-four possible, and equally prob- 
able, matings. But, as before, some of these matings give identical 

o Professor Bateson uses the symbol for first-generation hybrids. The F is the 
initial letter of the word "filial." Hence this symbol means "first filial generation." 
Bateson denotes the second and later generations of a hybrid by F2, Fg, etc. Likewise, 
he denotes parental generations as follows: 

Pi=parent8 of the hybrid. 

P2=grandparents of the hybrid. 

P3=great-grandparents, etc. 

165 



LAW OF EECOMBINATION. 21 

results; for instance, W Nc X WnC £ind WNC X Wnc both give 
WWNnCc. There are, however, twenty-seven different combina- 
tions amongst the sixty-four matings; these, together with the 
number of matings in which each occurs, are shown in Table 11. 

Table II. — The twenty-seven different types in the second generation of a hybrid between 
a ivinter wheat, W, with weak straw, n (absence of nonlodging character), and open chaff 
c (absence of closed chaff), and a spring wheat, w (absence of winter character), with stiff 
straiu, N (nonlodging) , and closed chaff , C. 



Serial 


Propor- 


Formulse of 


Serial 


Propor- 


Formulse of 


Serial 


Propor- 


Formulae of 


No. 


tions. 


the types. 


No. 


tions. 


the types. 


No. 


tions. 


the types. 


1 


1 


WWNNCC 


10 


2 


WwNNCC 


19 


1 


wwNNCC 


2 


2 


WWNNCc 


11 


4 


WwNNCc 


20 


2 


wwNNCc 


3 


1 


WWNNcc 


12 


2 


WwNNcc 


21 


1 


WW NNcc 


4 


2 


WWNnCC 


13 


4 


WwNnCC 


22 


2 


wwNnCC 


5 


4 


WWNnCc 


14 


8 


WwNnCc 


23 


4 


WW Nn Cc 


6 


2 


WWNncc 


15 


4 


WwNncc 


24 


2 


WW Nncc 


7 


1 


WWnnCC 


16 


2 


WwnnCC 


25 


1 


wwnnCC 


8 


2 


WWnnCc 


17 


4 


Wwnn Cc 


26 


2 


wwnn Cc 


9 


1 


WWnncc 


18 


2 


Wwnncc 


27 


1 


wwnncc 




16 






32 






16 





16+324-16=64. 

Types Nos. 1, 3, 7, 9, 19, 21, 25, and 27 are homozygote, and will all reproduce true to seed. Type No. 1 
was the type sought in this cross. 



The first nine of these twenty-seven types, constituting one-fourth 
of the whole generation, are pure winter wheat ( TFTF), the next 
nine (Nos. 10-18), constituting one-half of the generation, are hybrids 
between winter and spring (Ww), while the last nine, constituting 
one-fourth, are pure spring wheats (ww). Each of these three 
groups of nine types is subdivided in like manner into one-fourth 
pure nonlodging, one-half hybrid between lodging and nonlodging, 
and one-fourth pure lodging. Thus, the first three types are all pure 
winter and pure nonlodging; these three types constitute 4 sixty- 
fourths of the generation, or one-fourth of the first group of nine. 
The second group of three are all pure winter, but- hybrid with 
reference to the lodging character; these three constitute 8 sixty- 
fourths of the generation, or one-half of the first group of nine, etc. 
Thus, each of the three groups based on the winter-spring character 
pair is subdivided into three groups based on the straw character, 
thus giving nine groups based on these two character pairs. Each 
of these nine is similarly divided into three t5rpes, based on chaff 
character. This gives in all twenty-seven different combinations. 
Of these twenty-seven combinations, eight are seen to be homozy- 
gote with reference to all three character pairs. This means that 
these eight are pure bred as far as these characters are concerned 
and will show these characters in all their progeny. The other nine- 
teen types are heterozygote, or cross-bred, with reference to one or 
more of the character pairs, and will thus not reproduce true to seed. 

165 



22 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 

The one combination which was sought in this cross is type No. 1 
of Table 11. This type constituted only 1 sixty-fourth of the second 
generation of this hybrid. It is the combination WWNNCC, which 
is pure winter wheat, nonlodging, with tightly closed chaff. The most 
undesirable type, wwnncc, also occurred once in sixty-four times — that 
is, it was a spring wheat which lodged and had weak chaff. Further 
mention of this new type of wheat will be made later in discussing 
the application of the principles to plant breeding. 

We may now state the law of recombination as follows : In the sec- 
ond generation of a hybrid there tends to occur every possible com- 
bination of the original parent characters. 

We may further add to this law that every one of these combina- 
tions will, if the second generation is numerous enough, occur in some 
individuals in homozygote form, and will thus be firmly fixed and 
reproduce true to seed. 

Although all the possible combinations will occur in the second 
generation of a hybrid (that is, provided the second generation is 
numerous enough to permit them to occur) unfortunately most of 
them will be mixed with other combinations that have the same 
external appearance but very different hereditary tendencies. This 
is due to the fact of dominance. For instance, the homozygote form 
WWNNCC of Table II can not be told by inspection from the form 
just following it (WWNNCc) or from several other of the twenty- 
seven combinations. One way to overcome this difficulty in a self- 
pollinated species is to save the seed of each 'second-generation plant 
separately. If the species is one that is not self-fertilized, but one 
which can be artificially self-fertilized, we can accomplish the segre- 
gation of the desired type by artificial self-fertilization of all the sec- 
ond-generation individuals that appear to be of the type desired and 
by planting their seed separately. When the next generation matures 
it will be seen which of them have reproduced true to type. The 
seed of these may be saved, and thus form the basis of a new and 
fixed variety in self -fertilized species and in such open-fertilized 
species as will endure such self-fertilization. 

A very beautiful illustration of the law of recombination is seen in 
the work of Professors Price and Drinkard, of the Virginia Agricul- 
tural Experiment Station, in their experiments with hybrid tomatoes. 
Two varieties were crossed which differed in three respects, namely, 
one variety had green leaves, the other yellowish green ; one had red 
fruit, the other yellow; one had pear-shaped fruit, the other round. 

"The writer discovered this law independently in 1901. See Bulletin No. 115, 
Office of Experiment Stations, U. S. Dept. of Agriculture. 
165 



FLUCTUATING VARIATIONS. 



23 



In the second generation of this hybrid every one of the eight possible 
combinations of these three pairs of characters was found, as follows: 



Serial No. 


Leaf color. 


Fruit color. 


Fruit shape. 


1 


Green 


Yellow 


Round. 

Pear-shaped.- 

Round. 

Pear-shaped. 

Round. 

Pear-shaped. 

Round. 

Pear-shaped. 


2 


Green 


Yellow 


3 


Green 


Red 


4 


Green 


Red 


5 


Yellowish 


Yellow 


6 


Yellowish 


Yellow 


7 


Yellowish 


Red 


8 


Yellowish 


Red 









Two of these eight types were like the original parent varieties, the 
other six were new. This case illustrates well the power a knowledge 
of the law of recombination puts into the hand of the breeder. Breed- 
ers have unconsciously used this law since breeding first became an 
art, but a knowledge of the principles involved now enables them to 
accomplish desired results much more quickly and surely than was 
formerly the case. 

FLUCTUATING VARIATIONS. 

From what has been said concerning the law of recombination it 
is easy to see that in a species which naturally cross-fertilizes in the 
field we are continually getting new combinations of hereditary char- 
acters. For instance, in a cornfield hardly any two plants can be 
found that carry exactly the same combination of hereditary charac- 
ters. If we should take a single grain of corn and plant it where it 
can not cross-fertilize with another its progeny would break up into 
types somewhat as shown in Table II, except that, instead of stopping 
with twenty-seven different types, each of these would be subdivided 
into three others, and each of these again subdivided in the same way, 
and so on within the limits of the number of separate and independ- 
ent hereditary characters for v/hich the grain we started with was 
heterozygote. Generally speaking, only a comparatively few of these 
characters will be important to the breeder, so that the others may 
be neglected. But we must not overlook the fact that in the main 
the remarkable fluctuations of characters seen in a cornfield are due 
to this recombination of characters from year to year. On the other 
hand, if we take a single grain of wheat and plant it, then save every 
seed of it for planting, the plants produced in the second generation 
would, ordinarily, be exactly alike in so far as their combinations of 
hereditary characters are concerned. This is because under ordi- 
nary field conditions wheat is self -fertilized, and a field of wheat in the 
main consists of plants that are completely homozygote with re.fer- 
ence .to every one of their hereditary characters. When we do get 

165 



24 APPLICATION OF PKINCIPLES OF HEREDITY TO BREEDING. 

a plant which is completely homozygote in all its characters, then it 
will transmit the same form of every character to all its offspring, and 
we have eliminated all variations due to recombination of characters. 
There will still be differences between the plants grown from the same 
seed, but these differences will be due to environmental influences, 
such as differences in available moisture, plant food, sunlight, and the 
like. It is highly important to make this distinction between indi- 
vidual variations which are due wholly to environment and those 
which are due to recombinations of hereditary characters. It will be 
seen later that so far as experimental evidence goes there is much 
reason to believe that selection of those fluctuations which are due 
wholly to environment as a rule has no effect whatever in changing 
the hereditary characters of the plant. On the other hand, in those 
plants which are not homozygote in all their characters, as is. practi- 
cally always the case in a species that regularly cross-fertilizes, there 
will be variations due to recombinations of different characters, and 
selection will have a marked effect in species of this kind. The effect 
of selection on fluctuating variations has been much confused because 
of the effect produced by mass selection in mixed populations of fixed 
forms like wheat, which effect will be further discussed later. In 
wheat, and other self-fertilized species, individual selection^that is, 
selection in which we keep the progeny of each mother plant sepa- 
rate — soon proves that we can not modify these fixed strains by 
selection; that is, generally speaking. The facts have further been 
confused because of the effect which either mass or individual selec- 
tion has in gradually changing the character of cross-fertilized crops 
like corn. In these cross-fertilized crops either mass selection or 
selection annually to a single mother plant causes a gradual change 
in the direction of the selection. But when we eliminate the effect 
of the law of recombination, which occurs continually in cross-fer- 
tilized forms, and practice selection annually to a single mother 
plant, we find that it is apparently impossible to produce modifica- 
tion by selection, except in rare instances. The investigations on 
which this reasoning is based will be given later in these pages. 

The amount of investigation which this subject has received can 
hardly be said to be sufficient to settle it for all cases, for there are a 
few exceptional cases which do not behave in the usual way and 
which are not understood. In the main, however, the investigations 
all agree. The first work bearing strictly on the effect of selection 
on forms from which all variation other than fluctuating variation 
due to environment has been eliminated was done by Prof. W. 
Johannsen, of Copenhagen, on beans and barley. This work will 
be referred to more in detail when we come to consider the effect of 
selection on self-fertilized species, as will also the remarkably accu- 

165 



EtJNNmG OUT OF VAEIETIES. 



25 



rate work of Doctor Nilsson, whose work at Svalof, Sweden, is so 
well known through the writings of De Vries. Some of the work on 
vegetatively propagated species will be given in dealing with the 
effect of selection on this class of plants. 

RUNNING OUT OF VARIETIES. 

It is quite generally believed that there is a tendency for vegeta- 
tively propagated varieties to ''run out." This subject has received 
much discussion but very little careful investigation. There is no 
question that in many species such varieties do lose vigor and become 
practically worthless after a few years. Carnation growers all agree 
that varieties of these plants are short lived. Varieties of carna- 
tions seldom retain their vigor a dozen years. It is generally be- 
lieved that varieties of potatoes retain full vigor only for a few decades. 
With no selection this is undoubtedly true. We do not know just 
what effect careful selection to maintain yield might have on the 
length of life of a variety of potatoes. It is certain, however, that 
the old Peachblow potato, so popular half a century ago, has been 
maintained in full vigor by selection to the present time. Mr. E. H. 
Grubb, of Colorado, one of the leading potato growers of that State, 
is now growing this variety and finds it an excellent yielder. It is 
probable, however, that potato varieties do tend to run out. The 
same may be said of apples, but, as in the case of potatoes, definite 
investigations on this point are lacking. 

On the other hand, there are species that have completely lost the 
power of producing seed, as, for example, the banana. These have 
been propagated vegetatively for ages without loss of vigor. But 
we can not say that the same variety persists indefinitely, because 
the facts are wanting. It is probable that among vegetatively 
propagated races we may find every gradation between races which 
run out very quickly and those which remain vigorous indefinitely. 

A good many species of plants produce seeds parthenogenetically," 
as the dandelion, certain species of Hieraceum, etc. These are 
among our most vigorous weeds. How long they have propagated 
asexually we do not know. They may, however, be cited as in- 
stances of plants which apparently retain their vigor through long 
periods without recourse to sexual propagation. 

What has been said of races propagated vegetatively applies 
equally to those which are habitually self-fertilized, with perhaps 
the difference that in most such species there may occasionally occur 
cases of cross-fertilization. It is quite generally assumed that 
self-fertilized races tend to run out. There is some evidence that 

That is, without fertilization. In these plants the reduction division is omitted 
in those mother cells which develop into seeds. 
165 



26 APPLICATION OF PEINCIPLES OF HEREDITY TO BEEEDING. 

such is the case. This is especially true of varieties of wheat. A 
single variety seldom retains its supremacy in any given locality for 
half a century. Yet it is far from demonstrated that careful selec- 
tion of wheat varieties would not maintain vigor almost indefinitely. 
This whole question of the running out of varieties needs much 
further study before the last word can be said on the subject. 

Having outlined the main principles with which we have to deal 
in plant breeding, we may now proceed to a consideration of the 
different methods of breeding and selection and the application of 
the principles involved. 

SELECTION WITHOUT ARTIFICIAL CROSSING. 

We have already seen that close-fertilized and cross-fertilized 
species behave difi^erently under selection. In addition to self- 
fertilized and cross-fertilized species we must also consider the effect 
of selection on those varieties which are propagated vegetatively^* 
that is, from cuttings, grafts, tubers, etc., including all methods of 
propagation other than from seed. 

VEGETATIVE PEOPAGATION. 

In plants propagated vegetatively we have several kinds of varia- 
tion to consider, for the effect of selection on each of these is different. 
First, we have those fluctuating variations which are due wholly to 
environment, such as difference of food supply, moisture condi- 
tions, etc., which modify the individuals of a generation but which 
are not hereditary. As we have already seen, such investigations 
as have been made on this subject indicate that in nearly all cases 
of fluctuating variation due to environment selection is entirely 
without permanent effect in changing the plant from year to year. 
In Bulletin No. 127 of the Illinois Agricultural Experiment Station 
Dr. E. M. East made a careful survey of all the literature he could 
find relating to the effect of selection on these fluctuating variations 
in potatoes. He concluded that it is not proved that selection can 
change these variations, though the question is left in some doubt. 

Prof. H. S. Jennings, of Johns Hopkins University, has during 
the past few years made some investigations on the unicellular animal 
Paramecium which must rank among the most important biological 
work that has been done, at least in the field of experimental evo- 
lution. While Paramecium is an animal, it propagates for hun- 
dreds of generations by simple division, and hence there is every 
reason to suppose that the principles applicable to Paramecium are 
applicable to plants which are propagated vegetatively. Jennings 
gives an excellent summary of his work in the American Naturalist 
for June, 1909, where reference will be found to the original technical 

165 



SELECTION WITHOUT AKTIFICIAL CKOSSING. 



27 



publications. First, he studied abnormal individuals to see whether 
or not their abnormalities were inherited. In many of his cultures, 
individuals could be found that had various peculiarities. Although 
he sought patiently for some peculiarity of this kind that might be 
inherited and although he found many such pecuharities, in no case 
did he find one that is inherited in the proper sense of the word. 
The peculiarities in question never appeared in both of the individ- 
uals resulting from a division. Thus there was no tendency for 
them to multiply and spread over the race. Concluding his discus- 
sion of such cases, Jennings remarks: ''Examination of a large num- 
ber of cases in Paramecium shows that these untypical characters 
are never reproduced in the young." 

Jennings also found that the descendants of a single individual 
varied greatly in size. This suggested the idea that by selecting 
continually from the largest and the smallest, two races could be 
developed which would differ in size, although descended from the 
same original individual. This experiment was carried on for a 
very long series of generations, one line consisting of the largest 
individuals that could be found and of their largest progeny, the 
other of the smallest individuals and their smallest progeny. At 
the end of the experiment the two lines were brought under the 
same environmental conditions and within a very short time the 
average length of the two types became identical. This led Pro- 
fessor Jennings to remark that ''Selection within a pure race is of 
no effect on size," and again, "Neither selection nor environmental 
action changes the size of the pure race." 

This investigator found eight distinct types of Paramecium in a 
group which was previously supposed to consist of two species differ- 
ing in size. Figure 1 illustrates the relative sizes of the individuals 
in these eight races. It is seen that even the extreme forms overlap, 
and it was found that if the smallest individual of the largest race be 
selected and its progeny grown with continual selection from the 
smallest individuals to be found, no matter how long such selection 
was continued the progeny of this small individual would soon cover 
the whole gamut of variation of the race to which it belonged, and 
the same was true for each of the other races. Speaking of the 
effect of selection on such a species, Jennings says: 

How will selection act on such a complex species? As we have seen, selection 
within a single race is without effect. But if we make selections among the indi\'iduals 
of a mixed collection of races, such as figure 1 shows, we reach most instructive results. 
By making our selections in the proper way, we for a time make steady progress 
toward a certain goal. We will suppose that we do not know of the existence of these 
races; this is the case with most experiments in selection. From the species as a 
whole, as shown in figure 1, we will select for increased size. Let us follow the old 
plan of selecting many individuals showing the desired character; we will preserve 
all specimens above the mean size of the entire collection; that is, we divide the 
165 



28 APPLICATION OF PKINCIPLES OF HEREDITY TO BEEEDING. 



collection at x — o:, rejecting all those to the right. By so doing it is evident that we 
exclude all specimens of the two smallest races c and i, while preserving the majority 
of the specimens of the larger races. Allowing these to propagate, we of course get 
a mixture of the remaining larger races. Hence the mean size of the whole collec- 
tion will be greater than at first. Selecting again those above the mean size of this 
lot, we drop out another small race, and the mean of the collection as a whole again 
rises a little. We are making good progress in the improvement of our species. By 
taking successive steps of this character, dropping out the smaller races, first partly, 
then completely, one after another, we can for a long time continue to improve by 




a 

X 

Fig. 1.— Graphic illustration of the range of fluctuations of each of the eight pure races of Paranieciuin 
studied by Jennings. (Reproduced from the American Naturalist.) 

selection, but finally we reach a stage in which all but the largest race have been 
excluded. Thereafter we can make no further progress. In vain we choose for 
breeding the largest specimens of the lot; all belong to the same race, so that all pro- 
duce the same progeny. Selection has come to the end of its action. * * * 

Selection here consists simply in isolating already existing races. It produces 
nothing new. * * * 

Systematic and continued selection is without effect in a pure race, and in a mix- 
ture of races its effect consists in isolating the existing races, not in producing any- 
thing new. 

165 . 



SELECTION WITHOUT AKTIFICIAL CROSSING. 



29 



Similar work with identically the same result has been done on 
hydra by Elise Hanel^ and byM. A. Barber^ on yeasts and bacteria. 
In Barber's work there were some exceptional cases which will be 
mentioned later. He found many races of each, but each race was 
constant, with the exceptions noted below. Long-continued selec- 
tion had no effect in changing one of these races. Barber also studied 
individuals having various peculiarities. While the vast majority 
of these peculiarities behaved exactly as Jennings found them to 
do in Parameciimi, he did find a few cases within a pure race (that 
is, in the descendants of a single individual) that transmitted their 
peculiarities to their descendants. Here we have actual evolu- 
tionary change in a race. Races of yeast were produced having 
cells of different form from the parent type and races of bacteria 
composed of longer rods than the parents, but such cases were 
extremely rare. Thus we must assume that there are occasionally 
permanent evolutionary changes. As to the amount of change in such 
cases we can get some information from Jennings's races of Para- 
mecium, assmning, of course, that the differences between the various 
races have come about by evolutionary change. The difference 
between the average size of the two smallest races of Paramecium 
studied by Jennings was only 0.00028 inch, yet the progeny of any 
individual, large or small, in either of these two races, accurately 
maintained this difference between the races. The important point 
in all this is that when we are dealing with individuals of a pure race, 
or, as Webber calls them, a ''clonal" race of variety — that is, indi- 
viduals descended from a single individual b}^ vegetative propa- 
gation — except for those very rare cases in which positive evolu- 
tionary change occurs the fluctuating differences between indi- 
viduals have absolutely no bearing on the evolutionary process. 
According to Jennings there seems little doubt that this is true for 
organisms in general. He says: 

In Paramecium, in the extensive study of many races for hundreds of generations 
by exact statistical and experimental methods, not one single instance was observed 
of variation in the sense of an actual change in the race. 

So far as the e^'idence goes every race is essentially the same throughout the work 
and may have been the same for unnumbered ages. 

Jennings emphasizes the fact that real evolutionary changes do 
not occur often or easily. ''The fundamental constitution of the 
race is resistant to all sorts of influences. It changes only in excess- 
ively rare instances and for unknown causes." 

In summarizing his conclusions, Jennings makes the following 
statement: "Until some one can show that selection is effective 
within pure lines it is only a statement of fact to say that all the 
experimental evidence we have is against this." 



« Cited by Jennings in American Naturalist, June, 1909, 

165 



30 APPLICATION OF PRINCIPLES OF HEKEDITY TO BREEDING. 

The following statement is made by Dr. Raymond Pearl and Mr. 
Frank M. Surface in Bulletin No. 166 of the Maine Agricultural 
Experiment Station: 

There is a rapidly accumulating mass of evidence that the chief, if not the entire, 
function of selection in breeding is to isolate pure strains from a mixed population. 
It is found in actual experience impossible to bring about by selection improvement 
beyond the point already existing in the pure (isolated) strain at the beginning. 

These writers do not here distinguish between the effect of selec- 
tion in self-fertilized and cross-fertilized species, but what is said does 
apply to close-fertilized species strictly, where hybridizing is not 
practiced, and with certain limitations it also applies to cross- 
fertilized species, as will be seen later. 

If the conclusion that selection of fluctuating variations is without 
effect is correct, then it follows that after we have by trial found the 
best individuals in a crop propagated vegetatively we have gone as 
far as selection enables us to go, except as immediately stated below. 

But there is a second type of variation in vegetatively propagated ^ 
crops which can be affected by selection. Each individual plant is 
endowed with a certain number of hereditary characters. These 
characters may or may not come to complete development under 
given environmental conditions, or some of th-em may reach com- 
plete development while others may fail to do so. In so far as this 
failure to develop is due solely to environmental conditions selection 
is without power to modify the crop. But it would appear that 
from time to time, or perhaps more or less continuously, changes are 
going on with reference to these hereditary characters by which 
their tendency to develop under given conditions changes; so that 
in a crop like potatoes we may in time get a good many varieties 
from the descendants of a single individual. But these varieties, in 
the main, arise by certain hereditary characters becoming latent or 
possibly in some cases disappearing altogether. Again, it may be 
that the tuber with which we start a race may have a good many 
latent characters in it whose tendency to develop may subsequently 
increase, so that occasionally we get a variety which differs from that 
with which we started by the development of certain characters 
which were not patent in our original stock. For instance, a white 
variety may produce tubers with colored skin. Color is especially 
likely in white varieties to occur in the vicinity of the eyes of the 
tuber. 

The more usual variation which occurs in such cases is for charac- 
ters that are present to become latent, so that we are more likely to 
get light color or white from colored stock. 

What has been said about variation in vegetatively propagated 
plants appKes also to bud variations, or the so-called ^'bud sports." 

165 



SELECTION 



WITHOUT 



ARTIFICIAL CKOSSIXG. 



31 



In nearly all cases these sports dili'er from the plant on which they 
originated by lack of characters that are visible in the mother plant. 
Occasionally, however, the reverse is true. But when a new charac- 
ter appears in a bud sport it is in nearly all cases a character common 
to the species, which was presumably latent in the mother plant. 

The Ethel ^laule dahlia furnishes what appears to be an example. 
This is sold as pure white. Mr. TT. A. Andrews, of Washington, 
D. C, has grown this dahlia for four years. Last year (190S) one of 
the plants produced flowers having a decided pink tinge, especially 
in the center of the flowers. This year he has several of the plants 
produced (by division) from the pink-flowered one of last year, and 
all of them show the pink color. All the plants of this variety in 
Mr. Andrews's garden have been propagated by division from a single 
plant obtained four years ago. Presumably the pink color is latent 
in the original stock and has been partially revived in these pink- 
flowered individuals. 

In those vegetativeh" propagated plants where variation occurs by 
hereditary characters becoming latent — and this type of variation 
seems to be quite common, especially in potatoes — selection of seed is 
of great importance. In this case selection enables the breeder to 
keep his stock up to standard, at least much longer than woidd be the 
case without selection, and where variation occurs by the develop- 
ment of characters which were previously latent it enables him to 
preserve such variations when they are of value. 

The residts of the application of the principles here stated to the 
selection of seed potatoes have been in some cases quite marked. 
For instance, a potato grower in Michigan some years ago began the 
practice of digging by hand enough potatoes for seed and saving only 
those hills that had six or more merchantable tubers and no small 
tubers. TTlien he first began this practice he found only sixteen 
hills out of each hundred dug that came up to his standard; but after 
he had continued the practice for five years the number of such hills 
had risen to seventy in a himdred. Under the direction of Mr. L. G. 
Dodge, of the Office of Farm Management, several Xew England 
potato growers have been applying these principles for two years 
past. The first year there was an average of about eight hills per 
hundred that came up to standard. The second year from seventeen 
to twenty hills met tlie conditions. This is as far as the experiment 
has proceeded at the present time. Some work done on potatoes by 
Mr. C. W. Waid. of the Ohio Agricultural Experiment Station, has 
given similar results. In this work, starting with the same original 
lot of tubers, three strains were grown, as follows: (1) Seed from 
high-yieldiiig hills, (2) seed from low-jdelding hills, and (3) imselected 

81599°— BiiL 165—11 3 



32 APPLICATION OF PRINCIPLES OF HEEEDITY TO BREEDING. 



seed. Taking the yield of the unselected seed as a basis, the results 
were as follows: 



Source of seed. 


1904. 


1905. 


1906. 


1907.a 


1908.& 


From high-yielding hills 


122 


127 


147 


125 


171 


From unselected hills 


100 


100 


100 


100 


100 


From low-vielding hills 


70 


55 


77 


66 


91 



a See Report, American Breeders' Association, vol. 3, p. 191 et seq. 
b See Circular No. 90, Ohio Agricultural Experiment Station. 



SELF-FERTILIZED SPECIES. 

The effect of selection on self-fertilized species, such as wheat, 
barley, and oats, is essentially the same as it is on species propa- 
gated vegetatively. Doctor Nilsson in his remarkable work at 
Svalof , Sweden, has many times taken an unselected lot of seed from 
some cultivated variety of wheat or oats and planted each seed indi- 
vidually to study the character of the plants produced. He rarely 
finds two plants exactly alike. But when he saves the seeds from 
these plants separately the next year the progeny of each plant is, as 
a rule, found to be so much like the parent plant as to be indistin- 
guishable from it except for such fluctuations as may be due to en- 
vironmental influences only. 

Occasionally in work of this kind a plant is found which is not 
homozygote in all its characters. In other words, it is not abso- 
lutely pure bred. These plants split up in the next generation 
according to the law of recombination. Furthermore, their presence 
indicates that there is occasionally cross-fertilization in wheat and 
oats, so that ultimately in a wheat field there may be found prac- 
tically every possible combination of all the characters present in 
the field, and in tim.e every one of these combinations will come to 
exist in some individuals in homozygote form; for, as will be seen 
later, a self-fertilized plant tends to split up into all the fixed forms 
which can be made from the various combinations of the characters 
present in it. 

Neglecting for the present the occasional cross-fertilizations in a 
field of wheat or oats and the resulting heterozygote plants that are 
produced in this manner, which will be considered under the next 
heading, selection without cross-fertilization in self-fertilized species 
can have no effect except to enable the breeder to find those indi- 
viduals which are best among the population with which he is deal- 
ing. After he has found these individuals he can not improve them 
by selection. On the other hand, he may be able to hold them up 
to a high standard by means of selection, for presumably, as in the 
case of potatoes, the hereditary characters present in wheat may 
change in their tendency to develop. Especially may characters 
that are present get into the habit of failing to develop and thus 

165 



SELECTION WITHOUT ARTIFICIAL CROSSING. 



33 



give rise to inferior plants in the progeny of what was originally a 
high-class individual. 

While experimental evidence for the above statements is not as 
plentiful as it ought to be, Doctor Nilsson has done so much work 
along this line that the propositions enunciated may be considered 
practically estabhshed. Professor Johannsen, of Copenhagen, has 
done a great deal of work of the same kind with exactly the same re- 
sults. At the 1906 Genetic Conference in London he said, '^In a 
population containing only one single type the selection of fluctua- 
tions has no action at all." Johannsen has several races of beans 
which he has grown pure for several years and which are fully ho- 
mozygote. He has fully tested the effect of selection on certain 
seed characters of these beans. Speaking of the results,^ he says, 
^'Selection for weight, for absolute length, or relative breadth has 
had absolutely no observable influence on these characters." Johann- 
sen has obtained similar results with barley. 

After what we have seen to be true in vegetatively propagated 
races it should not be surprising that similar results occur in self- 
fertilized races, for although such races go through the form of re- 
combining the characters when they produce seed, the two mem- 
bers of each pair of characters being exactly alike, we get no new 
combinations, so that reproduction by seed in completely homozy- 
gous strains differs little, if at all, in its results from vegetative propa- 
gation. 

The fact that in a field of a self -fertilized crop a very large major- 
ity of the plants are perfectly fixed in their hereditary characters 
and will reproduce themselves with almost absolute fidelity from 
seed has led a good many biologists to consider every one of these 
plants which differs from its neighbors in any way to be what they 
call ''elementary species." They overlook the fact that these 
forms are fixed simply because they are homozygote in all their 
characters and would behave in exactly the same manner whether 
the evolutionary changes that produced them are either very slow 
and gradual or occur suddenly at long intervals. 

The problem, then, in selecting self-fertilized plants is to find the 
best individuals and propagate from them. There are two ways 
of selecting such plants, which give somewhat different results. 
One of these we may term "mass selection," the other "individual 
selection." 

MASS SELECTION. 

Mass selection is that form of selection in which a number of supe- 
rior plants or parts of plants are chosen but their seed is not kept 

« Zeitschrift fiir Induktive Abstammungs- und Vererbungslehre. September, 
1908, p. 2. 
165 



34 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 

separate. It may happen that some of the selections thus made 
are superior because they have been grown under very favorable 
environment and that another year when the environment is less 
favorable they may prove to be very inferior. Others may prove 
to be superior under a wider range of environmental conditions, so 
that from year to year they will be superior. The next year mass 
selection would be mostly from those plants wiiich are what we may 
call permanently superior, together with a few of those which merely 
happen to be superior under the given conditions. The contin- 
uation of mass selection thus tends to improve from year to year 
the general character of the crop, but it does this by the gradual 
elimination of the progeny of those plants which are not superior 
except under very favorable conditions. This gradual improve- 
ment that occurs in mass selection has misled biologists and plant 
breeders generally into believing that selection could affect fluctu- 
ating variation. 

The effect of mass selection of self -fertilized crops is well illustrated 
by some of Prof. C. A. Zavitz's work at Guelph, Ontario, Canada. 
In his annual report for 1905 he gives the results of sixteen years' 
continuous mass selection on oats and barley. These are given 
below. For convenience, his results obtained by similar methods 
with potatoes are also given here. It will be noticed that mass 
selection has the same effect in seh-fertilized wheat and oats as in 
potatoes, which are propagated vegetatively. 



Table III. — Average yields hy four-year periods, in bushels per acre, of oats, barley, 
and potatoes, showing the effect of mass selection on self fertilized and on vegetatively 
propagated crops. 



Crops. 


1890-1893. 


1894-1897. 


1898-1901. 


1902-1905. 


Oats, average for 8 varieties 

Barley, average for 8 varieties 


Bushels. 
74 
50 
120 


Bushels. 
79 
54 
216 


Bushels. 
83 
63 
218 


Bushels. 

100 
63 
249 



The very marked effect in the case of potatoes is probably due to 
degeneration which had occurred in many vegetative strains before 
the selection began, the progeny of these degenerate strains being 
gradually eliminated by mass selection. 

The Mandscheuri barley, now so largely grown in Ontario, is 
descended from a single pound of seed obtained from Prussia in 
1889. Of this variety 567,000 acres were grown in Ontario in 1908. 
Since the introduction of Mandscheuri, the barley crop of Ontario 
has increased in value from $4,800,000 to $12,900,000, and this is in 
part due to the larger yielding power of this variety. The greater 
profit due to larger yields has caused an increase in acreage. 

165 



SELECTION WITHOUT ARTIFICIAL CROSSING. 



35 



INDIVIDUAL SELECTION. 

In the form known as individual selection we start with superior 
plants and keep their seeds separate. This enables us very quickly 
to determine which of the original selections are superior under a 
wide range of conditions, so that within a few years we can determine 
which of our original selections represent the best strains in the 
variety. Then by propagating from them and by continuously 
selecting to avoid saying any plants which may be deteriorating 
from hereditary characters becoming latent, we can maintain the 
variety at a high standard. But it must be remembered that we 
can not increase the superiority of a pure strain by selection except 
in those comparatively rare cases where characters that were latent 
in our original selection change in their tendency to develop and 
happen to increase the superiority of the strain. 

That latent characters may reappear in a variety is shown by the 
following facts. Sometimes, in varieties of potatoes having white 
skin, tubers are found which have purplish or red skin, at least over 
part of the surface, and especially about the eyes. Bud sports 
sometimes exhibit characters not apparent in the parent stock, but 
common to other varieties of the species. In Doctor Nilsson's 
work at Svalof, Sweden, black or yellow oats occur at wide intervals 
in white varieties. All these facts indicate that latent characters 
occasionally become patent. 

We have already referred to the effect of mass selection on barley 
at the Ontario Agricultural Experiment Station. Professor Zavitz 
has also used individual selection on varieties of this crop. In 1903 
he selected 9,972 grains of the Mandscheuri barley and planted them 
individually. Seed of 33 of these were planted separately in 1904. 
By 1908 all but three of these strains had been discarded. One of 
these, known as ^'O. A.C. No. 21, "which outyields the original variety, 
is now rapidly replacing the latter on Ontario farms. 

The selection at the Minnesota station, begun by Prof. Willet M. 
Hays, is individual selection. The seed of each plant, to serve as 
the original parent of a strain, is saved separately, so that the yield- 
ing power of pure strains is determined by several years' test of 
successive generations of their self -pollinating progeny. The best 
of these are finally brought into culture. This method enables the 
breeder to secure the best strains present in the seed with which he 
starts, or, as Professor Hays puts it, it enables the breeder to find 
those plants having the highest ''centgener" power; that is, the 
power of producing strains with maximum yields under the widest 
range of environmental conditions. Some of the wheats obtained 
in this manner at the Minnesota station have proved decidedly 
superior to the original mixed stock from which they were isolated. 

165 



36 APPLICATION OF PEINCIPLES OF HEREDITY TO BREEDING. 

In these homozygote forms, which constitute the major part of a 
tield of any crop which habitually self-fertilizes, there is little, if any, 
more variation than in plants which are propagated vegetatively. 

CROSS-FERTILIZED SPECIES. 

The eifect of individual selection on cross-fertilized species, such 
as corn, is very different from what it is in self -fertilized species. 
Here the plants chosen are more or less cross-fertilized with other 
plants and the seeds obtained from a single plant are not all alike 
in content of hereditary characters. Hence we may get distinct 
differences in the individuals grown from this seed. Either mass 
selection or individual selection in a crop of this character may make 
decided changes in it for the reason that, in the seed of every plant, 
combinations of hereditary characters will occur that are unlike 
those in the original plants selected. Some of these may be superior 
to the original plants. For instance, the plant with which we start 
may be heterozygote with reference to a particular character which 
we will call "AJ'' That is, it inherited from one of its parents the 
presence of this character and from the other its absence. Its 
formula with reference to this character would therefore be Aa. 
Such a plant will produce progeny one-fourth of which has the 
formula AA, one-half Aa, and one-fourth aa. Now, the combina- 
tion AA may be superior to Aa and aa, so that in the seed of our 
selection we may get something better than the plant selected. On 
the other hand, we may also get something not so good. Selection 
alone, therefore, enables us to make positive improvements in crops 
which regularly cross-fertilize. 

The work done on the corn plant at the Illinois Agricultural Ex- 
periment Station is perhaps the best illustration of the effect of 
selection on crops that cross-fertilize. Bulletin 128 of that station 
gives the results of ten years' selection of corn for high and low oil 
content and for high and low protein content. Some of these results 
are given in Table IV. 

Table IV. — Effect of selection in a cross-fertilized species. The figures of column 2 
give differences in percentage content of oil between two strains of corn of similar 
origin, one selected for high and one for low oil content. Column 3 gives similar differ- 
ences between two other strains selected, one for high and the other for low protein content. 



Years. 


Oil 
differences. 


Protein 
differences. 




Per cent. 


Per cent. 


1896 


0. 00 


0. 00 


1897. 


.67 


.55 


1898 


1.16 


.50 


1899 


1.82 


1.60 


1900 


2. 55 


2.98 


1901 


2. 6() 


4.08 


1902 


3. 39 


4.12 


1903 


3. 53 


4.52 


1904 


4.08 


5. 70 


1905 


4.71 


6.15 


1906 


4.71 


5.62 



1 0.^) 



SELECTION WITHOUT AETIFICIAL CROSSING. 



37 



Bulletin 132 of the same station gives the results of six years' selec- 
tion of corn for high and low ears. See Table Y. 

Table V. — Difference between two strains of corn selected for ears high or low on stalk. 



Years. 


Difference 
in height 
of ears. 


Difference 
in nuniber 
of inter- 
nodes in 
stalks. 




Inches. 


Number. 


1903 


13.6 


1.5 


1904 


12.0 


1.5 


1905 


21.7 


1.8 


1906 


31.1 


4. 1 


1907 


39.2 


3.3 


1908 


34.2 


4.0 



Another instance of the effect produced on a species which is par- 
tially cross-fertilized is seen in some interesting work of Professor 
von Riimker at the Breslau Experiment Station in Germany. This 
work was done on rye, in which species more or less cross-fertilization 
occurs. By continued individual selection for color of seed Pro- 
fessor von Riimker finally obtained several strains of markedly dif- 
ferent color. Yellow color was more difficult to fix than green. 
This is probably due to the compound nature of the yellow color 
from the Mendelian standpoint. Some interesting cases of correla- 
tion were found in this work. Green-colored seeds produced stronger 
stalks; brown seeds were less mnter hardy. It was found that the 
selection must be continued in order to maintain the characters for 
which the selections were made. It is doubtful if these results could 
have been obtained in a strictly self-fertilized species. 

The reason why selection produces these effects on cross-fertilized 
plants is seen in the follomng: Suppose we start with a corn plant 
that is heterozygote for yellow and white corn and for starchy and 
sweet corn characters. The presence of yellow may be represented 
by Y, the absence of yellow (that is, white) by y; the presence of 
starch-forming character by S, and its absence (that is, sweet-corn 
character) by s. Figure 2 shows the nine different types of corn 
which would be produced by the individuals. If, now, we plant all 
the seed produced by these nine types of corn and plant them where 
they can freely cross-fertilize, but where they will not cross with 
other kinds of corn, the next year we shall again get these same nine 
types, but not in exactly the same proportion. If corn were com- 
pletely cross-fertilized the proportion of these nine types would be 
approximately the same the second year, and each year thereafter, 
assuming, of course, that all the types are equally productive. This 
is illustrated in figure 2. On the other hand, if corn were completely 
self-fertilized, these nine types would behave as those sho^vn in fig- 

165 



38 APPLICATION OP PETNCTPLES OF HEREDITY TO BEEEDING. 



lire 4, where four of the nine types increase and five decrease until, 
in ten generations, the whole population consists of little more than 
an equal mixture of the four homozygote types. Corn, being largely 
but not completely cross-fertilized, would give a result intermediate 
between those shown in figures 2 and 4. If the amount of cross- 
fertilization in corn were fixed and definite, and if the average per- 
centage of cross-fertilization could be determined, it would then be 
possible to calculate the exact percentage of each type present in the 

population after the 
relation between the 
t}^es became con- 
stant ; also the rate at 
which each type 
approached its ulti- 
mate proportion of 
the population. 

Figure 2 shows 
what would happen 
if corn were com- 
pletely cross-fertil- 
ized. This figure will 
be understood when 
it is explained that 
the space between any 
two adjacent horizon- 
tal lines represents the 
proportion of the pop- 
ulation of the type 
represented by the 
formula in that space. 
Here each of the nine 
spaces maintains the 
same width from gen- 
eration to generation. This means that each of the nine t}^es present 
tends to remain in the same proportion from generation to generation 
under these conditions. 

One method of arriving at the data shown in figures 2, 3, 4, and 5 
is illustrated immediately below, for tliose cases in wliich there is 
complete cross-fertihzation, as in animals and in dioecious plants. 
Generation in hybrids wliose parents differ in one Mendehan char- 
acter only is IDZ), 2BR, IRR, where D represents the dominant and 
E the recessive character. Hence one-fourth of generation Fg con- 
sists of the type DD, two-fourths of type DR, and one-fourth of type 
RR. But since males and females are equally numerous, one-fourtli 



w 

J. J- 


ss 




'/f6 










YY 


Ss 




% 










YY 


SS 














Yy 


ss 
















Ss 














Yy 
















yy 


SS 














yy 


Ss 














vv 


ss 




'//6 











Generat/ons. 



10 



Fig. 2.— Graphic illustration of ten generations of corn with no se- 
lection, the first generation of which is 'YpSs.a The proportion 
of each type is indicated by the vertical space between the hori- 
zontal lines above and below the type symbols. Thus, YYSS 
is one-sixteenth of the whole population. Under the conditions 
mentioned each tjj>e tends to remain in the same proportion 
from year to year. 



« Complete cross-fertilization is here assumed, 
between tliose shown in figures 2 and 4. 

165 



The actual results are intermedial ( 



SELECTION WITHOUT ARTIFICIAL CROSSING. 



39 



of the males are of type DD, two-fourths of type DR, etc., and simi- 
larly for the females. Now a female of type DD may be fertilized by 
a male of any of the three types, and the probability that a given 
female shall be fertilized by a male of a particular type will .depend 
on the relative number of males of that type. Since one-fourth of 
the males are of type DD, the chance that any particular female shall 
be fertilized by a male of type DD is one-fourth. Since one-fourth 
of the females are of type DD, the chance that in a particular mating 
the female shall be of type DD is one-fourth. Hence the probability 
that a given mating shall be of type DD 9 X DD ^ is J X i = Tg^. 
The possible matings that can occur, and the relative probability of 
each of these matings, is shown in the left-hand column of Table A. 
Since the denominator is the same for all these matings, it is omitted 
for convenience. The products at the right of column 1 represent 
the relative frequency of each of the matings. Assuming all these 
matings to be equally productive, these same numbers represent the 
relative number of progeny from matings of each of the types of 
matings. 



Table A. — Method of determining the relative proportion of the various types in the 
progeny produced by the individuals of generation F.^, ivith cross fertilization. 



Matings and their relative frequency. 


Types of progeny and 
their relative fre- 
quency from each of 
the possible matings. 


DD 


DR 


RR 


DD X DD....1 X 1 = 1 ..- 


1 
1 






DD X DR.. .A X 2= 2 


1 
1 
1 
2 
1 
1 
1 




DD X RR 1 X 1 = 1 




DR X DD 2 X 1 = 2 


1 
1 




DR X DR.... 2 X 2 = 4 


1 
1 


DR X RR....2 X 1 = 2 


RR X DD....1 X 1 = 1 




RR X DR 1 X 2 = 2 




1 
1 


RR X RR....1 X 1 = 1 




Relative proportion in F3 






4 
1 


8 
2 


4 


Or 





In the next three columns the relative proportions of the different 
types of progeny in each of the matings are shown. By adding these 
columns it is seen that the ratio between the types in Fg is the same 
as it was in Fg. Hence it will tend to be the same in all subsequent 
generations, with cross-fertilization. With self-fertilization the 
results are different, as seen in Table B. 

165 



40 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 



Table B. — Method of determining the relative 'proportion of the various types of progeny 
produced by generations Fo and of a hybrid, with seJf -fertilization. 



T3T)es and their relative frequency. 





DD 


DR 


RR 


Types in F2: 

\DD...a2 


2 
1 






2DR 4 


2 


1 

2 


\RR 2 


Frequency of types in F3 






3 


2 


3 


Types in F3: 

3Z>Z)... 06 


6 
1 






2DR 4 : 


2 


1 

6 


ZRR 6 


Frequency of types in F4 






7 


2 


7 





Types of progeny pro- 
duced by each F3 
type, and their rela- 
tive frequency. 



a These numbers are taken instead of 1, 2, 1 (3, 2, 3 in F3) to avoid fractions in the next three columns. 

Here we see that the proportion of the types changes from gen- 
eration to generation, the homozygote types increasing while the 
heterozygotes decrease in relative frequency. 

It will be noticed that the sum of the relative frequencies in 
is 4 ; in F3, 8 ; and in F4, 1 6 ; and so on ; that is, in Fg it is 2^ ; in Fg, 2^ ; in 
F4, 2*; etc. In general, in Fn it is 2^. Similarly, the frequency of each 
of the types DD and RR is, in 

F. 



F3 F3 



F, Fe--. 

1 3 7 15 31 
Or, 21-1 22-1 2^-1 2^-1 2^-1 

The frequencies of all the types in Fn are — 

DD, 2^-1 - 1 
DR, 2 
RR, 2^-1-1 



Fn 



1. 



2x2^-l = 2^ 

The above calculations have been made for a single character pair 
for convenience. Exactly the same method may be pursued when two 
or more Mendelian pairs are concerned, except that where two pairs 
are involved we must deal with nine types instead of three, and where 
three pairs are involved we must deal with twenty-seven types, and 
so on. In general, the number of types appearing in Fg and later gen- 
erations is 3"^, where the n is the number of character pairs involved. 

To illustrate the effect of selection to a dominant type, let us con- 
sider the case of two character pairs, which we may denominate Aa 
and Bh, where A and B are dominant and a and h recessive. Gener- 
ation Fi produces nine types of progeny, which, with their relative 
frequencies, are shown in Table C. Here the selected types are those 
that have the appearance of the type AABB, namely, AABB, AABh, 
AaBB, and AaBh. It is seen that while type AABB is only one- 
sixteenth of generation F2, it constitutes one-fourth of the selection 
made in F3. By continuing Table C to ten generations, the data from 
which figure 3 was constructed may be obtained. 

165 



SELECTIOi^ WITHOUT ARTIFICIAL CROSSING. 



41 



Table C. — Method of determining relative frequency of the various types in the progeny 
of generation F.,, fith cross-fertilization, and selection to the dominant type, when two 
pairs of characters are involved. 



Matings. 


Relative Ireqaency of each type in the progeny of the various matings. 


AABB 


AABb 


AAbb 


AaBB 


AaBb 


Aabb 


aaBB 


aaBb 


aabb' 


Fi, AaBbX AaBb .... 1 X 1= 1 .... 10a 

F2, AABBXAABB...AXI= 1... 
AABBXAABb 1X2= 2... 


1 


2 


1 


2 


4 


2 


1 


2 


1 


1 
1 
1 
1 
1 
1 
1 
1 
1 

1 
1 
1 
1 
1 


















1 
















AABBxAaBB.. 1X2= 2... 




1 
1 












AABBxAaBb 1X4= 4... 


1 
1 
2 
1 
2 




1 










A ABbxA ABB.... 2X'i= 2... 
AABbXAABh 2X2= 4... 












1 














AABbXAaBB 2X2= 4... 


1 
1 
1 
1 

2 
2 
1 
1 
2 
2 


1 
2 










AABbX iaBb 2X4= 8 


1 


1 








AaBBxAABB....2Xl= 2... 
AaBBXAABb 2X2= 4... 








1 




1 











AaBBxAaBB 2X2= 4... 






1 
1 






AaBBxAaBb . 2X4= 8... 


1 
1 
2 
1 
2 




2 
1 
2 
2 
4 




1 




AaBbXAABB....AXl= 4... 
AaBbxAABb 4X2= 8... 








1 


1 






J.a£5X^a5jB . 4X2= 8... 


1 
1 


1 
1 


1 

2 




AaBbXAaBb.... 4X4=16... 
Frequency of types in F3 


1 


2 


1 


10 


16 


4 


16 


16 


4 


4 


4 


1 


Relative frequency of se- 
lected tjrpes in F3 


1 


1 






1 























a 16 progeny assumed to avoid fractions in the table. 



Since we do not know what percentage of cross-fertilization occurs 
in corn or any other open-fertihzed crop, it has been assumed in what 
follows that corn is completely cross-fertilized. The actual results in 
practice would be intermediate between figures 2 and 4 when there is no 
selection to type and between figures 3 and 5 when there is such selection. 

Referring again to figure 2, and supposing that we desire to select 
from this mixed population and perpetuate the type YYSS — that is, 
pure yellow starch-forming corn — we would at once discard the fol- 
lowing forms: YYss, Yyss, ijySS, yySs, and yijss. All of these would 
either be white or of the sweet type, or both. Discarding all these 
types that are not yellow starch-forming types, we would still have 
left the second, fourth, and fifth types shown in figure 2, all of which 
would be yeUow and would have starchy grains, because the presence 
of these two characters is dominant over their absence, and these types 
heterozygote for one or both of these characters can not be distinguished 
by inspection from type 1, which is the type we wish to perpetuate. 

Selecting for seed those plants which do have j^ellow, starchy grains — 
that is, types 1,2,4, and 5 of figure 2 — and planting them where they 
can cross with each other but not with other corn,^ the next year the pro- 
portion of each of the nine t}^es that would appear would be as shown 
in Table YI, column 3. If we make the same kind of selection again 
the next year the proportions of the nine types would be as in column 
4. Table YI shows the results that would be obtained at the end of the 
sixth generation by tins kind of selection in a cross-fertilized crop. 

« Complete cross-fertilization is here assumed. The actual results are intermediate 
between those shown in figures 2 and 4. 
165 



42 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 



Table VI. — Types and their percentages in the descendants of YySs for several genera- 
tions ivith cross fertilization and continued selection to type YYSS. Regarding YySs 
as the Jirst generation, we have: 



Types. 



Generations. 



2. 


3. 


4 


5. 


6. 


P. ct. 


P. ct. 


P. ct. 


P. ct. 


P. ct. 


6. 25 


19.8 


31. G 


41.0 


48.2 


12.50 


19.8 


21.1 


20.5 


19.3 


6. 25 


4.9 


3.0 


2.6 


1.9 


12.50 


19.8 


21.1' 


20.5 


19.3 


25. 00 


19.8 


14.0 


10.2 


7.7 


12.50 


4.9 


2.3 


1.3 


0.8 


6. 25 


4.9 


3.5 


2.6 


1.9 


12.50 


4.9 


2.3 


1.3 


0.8 


6.25 


1.2 


0.4 


0.2 


0.1 



YYSS 
YYSs. 
YYss. 
YySS.. 
YySs.. 
^Yyss... 

yyss... 
yySs.... 

yyss 



The type YYSS gradually increases until in the sixth generation 
it constitutes 48 per cent of the crop. This type, together with types 
YYSs, YySS, and YySs, all of which appear to be the same as 

YYSS, constitutes 
/^'erce/y/a^e of eac/? rype /n eac/?generc^^^ 94 5 ^^^^ 
^ared/^y/er^gf/^ of verf.ca/ /me cuf off l>y curves, ^i^^h generation. 

' I f 1 1 1 1 1 1 1 Thus in cross-ferti- 

^ lized crops mass se- 

lection to a given 
type gradually es- 
tablishes that type, 
not so rapidly, how- 
ever, as it does in 
self -fertilized species, 
after hybridization. 

Figure 3 shows the 
same thing graplii- 
cally for ten genera- 
tions. In this figure 
the space between 
the top curved Une 
and the horizontal 
line at the top of the 
diagram shows the 
proportion of the 
type Y YSS present 
from generation to 
generation. It is 
seen that in the sec- 
ond generation only a small proportion (one-sixteenth) of type Y YSS 
is present. In generation six, almost exactly half the population is 
YYSS, while in generation ten this type constitutes about 65 per 
cent of the whole. The proportion of type YYSs present from 

1G5 




2^ 3 

6enerat/ons. 

Fig. 3.— Graphic illustration of the effect of mass selection in cross- 
fertilized species. Selection for type Y YSS; types so arranged as 
to bring together those that resemble each other. The proportion of 
type Y YSS from generation to generation is shown by the space 
between the top horizontal line and the upper curved line. It is 
seen to increase from generation to generation. The proportion of 
type YYSs from generation to generation is indicated by the ver- 
tical width of the space })ctween the two upper curved lines. 
Similarly, the space beginning opposite each type formula indicates 
the proportion of that type present from generation to generation. 



SELECTION WITHOUT ARTIFICIAL CEOSSING. 



43 



generation to generation is indicated by the space between the two 
upper curved hues. It is seen that this type increases sHghtly until 
the fourth generation, after which it gradually decreases. The five 
types at the bottom of the figure decrease very rapidly from the sec- 
ond generation on, so that by the tenth generation they have almost 
disappeared, and the population is made up almost entirely of yellow, 
starch-producing types which are either pure (type YYSS) or 
heterozygote (types YYSs, YySS, and YySs) for one or both of the 
characters with which we are dealing. Table VI and figure 3 thus 
illustrate the effect of mass selection on crops which cross-fertilize, 
and this effect is seen to be a gradual approach toward the type 
selected. 

Evidently the limit of the effect of selection in a case of this Idnd 
is reached when practically the whole crop is homozygote for the 
character selected. No further advance can be made by selection, 
and it must be remembered that in order to hold the crop at this 
high degree of excellence selection must be continued, because here 
and there plants will vary by certain characters becoming latent and 
thus reducing the general average of superiority in the strain. 

The problem of improving cross-fertilized species or varieties 
by selection to a particular type is complicated by the fact that 
in plants which ordinarily cross-fertilize we are apt to lose vigor 
when we get our plants too much alike. That is, these plants have 
been used to cross-breeding, and when we get our population very 
uniform, which is only another way of saying get them very close kin 
to each other, the yield is liable to run down because of this very 
uniformity. There is some question, therefore, whether in a cross- 
fertilized crop we should attempt to get great uniformity. 

We can partially overcome this difhculty by each year selecting 
the best individuals we can find and planting them in alternate 
rows so as to get as much cross-fertilization as possible. In corn 
the ear-row method of breeding permits this to be done. The 
methods adopted by some of the best corn breeders in the country 
at the present time are essentially as follows: Every year they go 
through the seed plot and also through the cornfield and select the 
best ears they can find to use in the seed plot the next year. As 
many ears are selected as there will be rows in the seed plot. In 
this way there will be the normal cross-fertilization in the seed plot 
with the resulting vigor that comes from cross-fertilization. Careful 
account is kept of the yield of each of the rows in the seed plot, 
so that the breeder may learn what ears selected the year before 
tend to produce the best yields. This enables him the next year 
when he goes to the seed plot or the cornfield to select seed to know 
what types to look for, and it is probably wise, from the standpoint 
of yield, to select each year two or three types, if not more, pro- 

165 



44 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 

vided they all yield well, so as to insure as much cross-fertilization 
as possible in the seed plot. 

The fact is, we know very little indeed about the relation between 
yield of corn and type of ear. Prof. A. E. Grantham, of the Delaware 
Agricultural Experiment Station, recently called the writer's atten- 
tion to the fact that in communities that have been unaffected by 
modern ideas about corn breeding, the best ears of corn are usually 
of the so-called slick" type. He suggests that this may be a case 
of the survival of the fittest. Farmers have from year to year 
selected sound ears for seed, paying little or no attention to type. 
The prevalence of slick ears may therefore represent a case of the 
survival of the fittest. In testing local varieties of corn unaffected 
by modern ideas of selection alongside of the improved varieties, 
Professor Grantham states that the local varieties yield about as 
well as the others. The writer can partially verify this statement 
for southwestern Missouri. On his own farm there is a variety of 
corn that has been grown there for at least thirty years. The best 
ears of this variety are decidedly slick; on good land it has yielded 
80 bushels per acre. A single year's test of one of the noted improved 
varieties in comparison with this local one indicates that the improved 
variety will outyield the other considerably, but it is important to 
note that the improved variety, although selected for excellence 
for fifty years, has never been selected for uniformity of type and 
has not a few slick ears in it. 

The amount of careful investigation that this question of relation 
of type to yield has received is wholly inadequate. It ought to 
receive careful attention at the hands of our best investigators. 

There is another method of breeding open-pollinated crops, like 
corn, that, while it has not been extensively tried, seems to deserve 
consideration. This method consists of maintaining two pure strains 
w^hich are not close kin, and raising each year seed which is a cross 
between these two varieties and using this seed for the field crop 
the next year. This method was first proposed by Dr. G. H. Shull^ 
in an article read before the American Breeders' Association at its 
Washington meeting in January, 1908. Shull's results have been 
confirmed by Dr. E. M. East, of the Connecticut Agricultural Experi- 
ment Station, whose work is referred to later in these pages. A simi- 
lar plan was recently proposed by Mr. G. N. Collins, of this Bureau, 
in a buUetin entitled ''The Importance of Broad Breeding in Corn."^ 

a See Report, American Breeders' Association, voL 4. 

& Since the above was written. Mr. Charles P. Hartley, of this Bureau, has called 
the attention of the writer to some recommendations made in 1893 and 1894 by Prof. 
G. E. Morrow and his assistant, Mr. F. B. Gardner, of the Illinois Agricultural Experi- 
ment Station. In Bulletin 25 of the Illinois station these investigators say: "The 
fact that increased yields can be obtained by crossing two varieties is pretty certainly 
165 



SELECTIOX WITHOUT ARTIFICIAL CROSSIXG. 



45 



A representative of the Office of Farm Management several 3'ears 
ago reported that m a certam community in one of the Western 
States farmers generally planted white and yellow corn in alternate 
rows in their seed patches. In this Avay they were sure of getting 
the vigor that comes from hybridization in the corn to be used for 
the general field crop the next year. Some of the farmers simply 
planted theu^ whole field m alternate rows of yellow and white corn, 
and the next year used seed selected out of this field for planting. 
The next year they would go back again and get pure yellow and 
pure white corn from some outside source and start over again, but 
this is a crude method which gets advantage of the hybridization 
only every other year. A better plan would be to get two good 
varieties of corn, both known to be adapted to the conditions, and 
plant a seed patch somewhat more than twice as large as needed to 
produce seed for the fields the next year, planting the two varieties 
in alternate rows in the seed patch. In one-half of the seed patch 
one of these varieties is detasseled and m the other half the other 
variety is detasseled. On both sides of the seed plot the detasseled 
stalks would bear only hybrid grains. On the other hand, the stalks 
that were not detasseled would be fertilized by pollen from stalks 
of the same variety, except in the middle of the patch, where there 
would be some cross-pohination between the two varieties. At har- 
vest time pure seed of the two varieties is selected for the next year's 
seed plot from the extreme sides of the plot, where there has pre- 
sumably been no cross-fertilization between the two varieties, while 
seed for the general field crop is selected from the detasseled stalks 

established, and a few farmers are changing their practice accordingly. This is quite 
easily done, by planting in one row one variety, and in the next another variety, 
and removing the tassels of the one as soon as they appear. The ears forming on 
the rows having the tassels removed will be fertilized with pollen from the other 
rows, thus producing a direct cross between the two A'arieties. The seed should be 
selected from the rows having the tassels removed, and the experiments indicate 
that it will pretty certainly give a larger yield than the average of the parent varieties 
when planted under like conditions. 

In a comparison of five crosses with the average yields of their parents the aA'erage 
increase in yield due to having crossbred seed in the experiments above referred to 
was 9.5 bushels. 

Again, in Bulletin 31 of the Illinois Agricultural Experiment Station the same 
authors make this recommendation: ■"Farmers can produce crossbred seed in con- 
siderable quantities in the following manner: Plant with one variety in one planter 
box and another variety in the other. Remove the tassels of one variety before they 
begin to shed pollen, and the shoots of the same will be fertilized with pollen from 
the other variety, thus producing a direct cross. The seed should be selected from 
the rows from which the tassels have been removed." 

In three out of four comparisons between crosses and their parents reported in the 
bulletin last referred to the crosses outyielded the parents, the average increase being 
2.3 bushels per acre in favor of the crosses. 
165 



46 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 

from the whole plot. This method would take advantage of the 
well-known vigor which arises when two varieties of corn are crossed. 

Shiill has recently proposed a plan somewhat similar to the above, 
and one which is really an improvement on it in one respect, but not 
so good in another.^ He suggests using two pure strains, planting 
one of them off by itself to get pure seed of it for the seed patches 
the next year, while at the same time another seed patch for growing 
field seed is planted of alternate rows of the two varieties. In this 
patch all the stalks of the variety grown alone in the other patch 
are to be detasseled. All the seed produced by the detasseled stalks 
will be heterozygote, while all the seed on the remaining stalks will 
be pure bred of the other strain from that planted alone in the 
smaller patch. The principal difficulty with this plan is that of find- 
ing isolated spots for two seed patches instead of for one, as in the 
plan in which the two varieties are both planted in alternate rows 
in one patch. In ShulFs plan the two strains could be kept prac- 
tically pure; in the other plan they would mix to a slight extent. 

HYBRIDIZATION AND SELECTION. 

We have been considering only the effect of selection without 
deliberate hybridization or cross-fertilization, except such as occurs 
naturally in certain of the crops considered. We shall now consider 
the application of artificial cross-fertilization and subsequent selec- 
tion to the art of improving farm crops. 

As in the case of selection alone, the methods and results differ 
for vegetatively propagated, close-fertilized, and cross-fertilized crops. 

VEGETATIVELY PROPAGATED CROPS. 

It is perhaps easier to secure the advantage of hybridization in 
vegetatively propagated crops, such as fruits, berries, potatoes, hops, 
and sugar cane, than it is from those crops that reproduce from 
seed. The reason for this is that some of the heterozygote forms 
which occur in the first generation of the hybrid may be highly 
valuable, and these heterozygote forms can be propagated true to 
type because they are not propagated from seed. For instance, sup- 
pose we cross two varieties of apples or potatoes and get in the first 
generation a plant from which can be made a valuable new variety. 
All that is necessary is to propagate this new variety by cuttings. 

In the case of potatoes very little hybridizing has been done. 
The seedling plants do not attain their full development until propa- 
gated from the tubers for two or three years. It is therefore nec- 
essary gradually to eliminate the poorer stocks and to grow for 
some time a good many of the forms which result from crossing, to 
see whether or not they are valuable. 

a See Report, American Breeders' Association, vol. 5. 

165 



HYBRIDIZATION AND SELECTION. 



47 



It must also be remembered that nearly all crops which are propa- 
gated vegetatively belong to the class of crops which naturally 
cross-fertilize; so that when we make a cross between two of them 
we are really crossing things that are themselves many times hybrid. 
For instance, a Baldwin apple tree is heterozygote for a good many 
of its characters. For this reason it produces many kinds of pollen 
and ovules, and when we use the Baldwin in a cross we get numerous 
different varieties in the first generation, while if the two varieties 
used in the cross were completely homozygote for all their char- 
acters all the first-generation hybrids would be alike. 

SELF-FERTILIZED SPECIES. 

The principles involved in the hybridization and subsequent 
selection to type of close-fertilized species, like wheat and oats, have 
already been stated in more or less detail, but some additional points 
need to be brought out. In utilizing hybridization in close-fer- 
tilized crops the first problem is to select suitable forms for hybridi- 
zation. Sometimes these forms are already at hand and well known. 
The real object to be accomplished by hybridization in this case 
is to make use of the law of recombination, by w^hich we can bring 
together in one variety certain valuable characteristics which are 
found scattered among two or more varieties which we may have at 
hand or can secure. Take, for instance, the problem which pre- 
sented itself, while the writer was agriculturist at the Washington 
Agricultural Experiment Station, in connection with wheat growing 
in eastern Washington. When the writer first went to Pullman, 
where the experiment station is located, the farmers had been grow- 
ing wheat in that section for twenty-five or thirty years. They 
had tested many hundreds of varieties, but found very few of them 
adapted to local conditions. Only three varieties had at that time 
come into general use, and these three were each more or less re- 
stricted to definite areas of different rainfall. For instance, where 
the rainfall was 10 or 12 inches almost the only variety grown at 
that time was Bluestem (not the hairy chaffed Bluestem of the 
middle Northwestern States). This was grown for two reasons. 
In the first place, it was one of the very few varieties found that 
would grow tall enough to cut with so little rainfall. In the second 
place, it produced a very superior quality of flour, and millers paid 
about 3 cents a bushel more for it than for other varieties. But 
this tall-growing variety could not be grown where the rainfall was 
20 inches or more, because it would fall down if the season was at all 
unfavorable. Where the rainfall was about 18 inches a variety of 
club wheat known as ''Red Chaff" was very widely grown, while 
81599°— Bui. 1G5— 11— 4 



48 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 

in those sections where the rainfall was 22 inches or more the pre- 
vailing variety was another club wheat known as Little Club." 

The growers of Red Chaff gave as their reasons for using this 
variety that it stood up better than Bluestem and yielded more 
than Little Club, while the growers of Little Club stated that this 
was the only variety they had ever found that would stand up and 
hold its grain under their conditions. 

It happens that all three of these varieties are spring wheats, 
but long experience has shown that sowing them in the fall would, 
in favorable seasons, produce yields 30 to 60 per cent greater than 
spring sowing. Hence, all three varieties were generally sown in the 
fall, but they would frequently freeze out to a greater or lesser ex- 
tent. There was therefore an insistent demand for winter wheat. 
The writer had collected from various parts of the world an exten- 
sive series of winter wheat varieties, and in 1899 a large number of 
these varieties had been grown for five years. Many of them were 
perfectly hardy and made enormous yields in favorable seasons, 
but they were inclined to straw-fall and to shatter their grain as soon 
as they were ripe; so that it did not seem advisable to recommend 
any of them to the farmers. 

At that time Mendelian principles were unknown in this country 
and had been forgotten in Europe, so that the writer had intuition 
alone to guide him in his attempts to produce a variety of wheat 
adapted to local conditions. By chance these intuitions proved to 
be correct and led to the discovery of the law of recombination pre- 
viously stated. Fortunately, the work proceeded from the beginning 
just as it would have done had the writer had full knowledge of the 
law of recombination, for the law was discovered in time to use it as 
soon as it could have been used in this work. 

Eleven of the best yielding winter varieties were crossed with the 
Little Club and the Red Chaff varieties (the Bluestem could not be 
successfully grown at Pullman, where the rainfall was about 22 
inches). Among the first-generation hybrids there were, therefore, 
eleven kinds. The seed of each hybrid plant was saved separately, 
so that the next year we had as many plots as we had hybrid plants 
the year before. 

The object of this hybridization work was to combine the winter 
hardiness of one class of varieties with the stiff straw and the tightly 
closed chaff of the other varieties. We now understand why this 
combination succeeded in every one of the crosses, so that from 
each of them resulted new and fixed varieties of wheat combining 
the characteristics mentioned. As was to be expected, some of the 
new varieties proved to be much more productive than others. . 

165 



HYBRIDIZATION AND SELECTION. 



49 



The writer severed his connection with the Washington Agri- 
cultural Experiment Station at the beginning of 1902, but his suc- 
cessor and his assistants continued the work with the hybrid wheats, 
and this work is still in progress, being conducted by Mr. C. W. 
Lawrence, of the Washington station. 

Three of these hybrid varieties (which had been fixed by proper 
selection, methods of which are outlined below) were distributed 
in small quantities to the farmers in the fall of 1907. after having 
been carefully tested at the experiment station as to their yielding 
power. In the fall of 1908, 39,000 acres of these new varieties were 
reported as being sown by the farmers in eastern Washington. In 
two more years there will doubtless be seed enough for all. 

The case just cited illustrates one in which hybridization furnishes 
a means of securing new and valuable varieties, namely, the case 
where certain valuable characteristics are found in different varieties 
and it is desirable to unite these characteristics. Such cases would 
exist at most plant-breeding stations. Doctor Nilsson, at Svalof, 
has made extended use of these principles in producing new varieties 
of cereals at his station. The first task is to select the varieties 
having desirable characteristics. Frequently some of the charac- 
teristics will be found in varieties that are otherwise worthless, so 
far as their use as field crops is concerned, so that in breeding work 
a variety should not be rejected because of a single weakness. It is 
legitimate to use an inferior variety in crossing, provided it has some 
valuable characteristics. 

In working with self-fertilized crops the principles involved for 
the first two years are exactly the same as those in cross-fertilized 
crops illustrated in Table VI. The first generation of the hybrid 
is heterozygote for all those characteristics in which the two parent 
varieties differ. Where the parents differ in a great many respects 
the problem becomes quite complex, for the number of types that 
will be produced in the second generation is equal to 2 ^, n being the 
number of points in which the two parents differ. Thus, when they 
differ in one particular, that is, when we have one pair of allelomorphs 
to deal with, there are two distinct types in the second generation 
of the hybrid. If the parents differ in two respects, we have 2^=4 
distinct types in ¥^ (the second generation of the hybrid). Three 
differences give 2^=8 types, and so on. The figures given above 
assume that in each pair of allelomorphs there is complete dominance, 
so that the heterozygotes can not be distinguished from the pure 
dominants. If the heterozygotes can be distinguished, then the 
number of visibly different types in the second generation is 3"^. 
In fact, in so far as their content of hereditary characters is con- 
cerned there are always 3 types in the second generation when the 

165 



50 APPLICATION OF PKINCIPLES OF HEEEDITY TO BEEEDING. 



varieties crossed differ in n particulars. In many cases we can 
neglect a great many of these differences, because they are immate- 
rial and deal only with those character pairs which are important. 
Table VI shows what occurs when the parents differ in two respects. 
The first generation is heterozygote with respect to both character 
pairs. In the second generation nine types occur in the proportion 
shown in Table VI. 

In the third generation we get a different result with self-fertilized 
plants from that obtained in cross-fertilized plants, because in self- 
fertilized species a het- 

Lengfhs of vert/ca/ //nes cutoff by carves s/?ovy 
proporf/'on of types /n eac/j ge/?e/xrf/o/7. 

Types. 

'WWCC 

WWCc 



^WwCC 
WwCc 

Irnvcc 

[jVwcc 

[wwCC 

\vwCc 

WWCC 




2 3 4 5 

Generat/ons. 



ero zygote when it 
makes seed breaks up 
into one-fourth pure 
dominants , one - half 
heterozygotes, and 
one-fourth pure reces- 
sives ; while with open- 
pollinated species the 
proportion of hetero- 
zygotes produced by 
heterozygotes will be 
larger than one-half, 
because of the intro- 
duction of foreign 
pollen. 

Let us first consider 
what occurs in self- 
fertilized species if we 
make no selection at 



Fig. 4.— Graphic illustration of ten generations of a hybrid in a self- 
fertilized species without selection to type: W, winter character; w, 
absence of TF(i. e., spring character); C, club character (short, thick q^}]^ amOng the progeny 
heads); c, absence of C (i. e., long-head character). Types arranged 
so as to bring together those that resemble each other. The space 
between the top horizontal line and the uppermost curved line shows 
the proportion of type TnrCC present from generation to generation, 
and the space beginning opposite each type formula shows the pro- 
portion of that type. The population ultimately consists almost 
entirely of the four homozygote types present. 



of the hybrid. Figure 
4 shows what occurs in 
this case. Suppose the 
plant with which we 
are dealing is a hybrid 
between a long-headed winter wheat and a club or short-headed spring 
wheat. In the second generation we get the usual nine types, as seen 
at the left of figure 4, where F= winter character, 'w; = spring character, 
C = club heads, and c = long heads. The nine types are arranged 
in figure 4 so as to bring together those which appear to the eye to 
be alike. Thus the types WWCC, WWCc, WwCC, and WwCc are 
all winter clubs, since W is dominant over w and C is dominant 
over c. WWcc and Wwcc are long-headed winter wheats, wwCC 
and wwCc are spring clubs, while wwcc is a long-headed spring type. 



HYBRIDIZATION AND SELECTION. 



51 



The relative proportion of each of these nine types for a series of 
generations, from the second to the tenth, is shown in the diagram 
of figure 4. For instance, the proportion of type WWCC is indicated 
by the space between the top horizontal line of the diagram and the 
upper one of the curved lines. The generations are indicated by 
the figures at the bottom of the diagram. In generation two the 
space for type WWCC is narrow, constituting only one-sixteenth of 
the second generation. But this type increases from generation to 
generation until by the tenth generation it is practically one-fourth 
of the population; that is, when there is no selection to type. 

Type WWCc is seen to decrease from generation to generation, 
as indicated by the space between the two upper curved lines. This 
space gradually becomes narrower, so that it has practically dis- 
appeared by the tenth generation. The space beginning opposite 
each type formula shows what happens to that type. It is seen that 
the four homozygote types WWCC, WWcc, wwCC, and wwcc gradually 
increase in proportion while all the heterozygote types decrease. By 
the tenth generation the whole population consists practically of the 
four homozygote types, each of them constituting practically one- 
fourth of the population. Only small amounts of any of the heter- 
ozygote types remain in the tenth generation. Of these four ho- 
mozygote types, two of them will be just like the two parents, as far 
as the characters we are considering are concerned. The other two 
will represent new combinations of the characters under consider- 
ation; the new types are (1) winter character with club heads and 
(2) spring character with long heads. We may therefore in such 
cases (i. e., with self -fertilized crops) secure our hybrid and plant 
its s.eed for several years without any selection at all, then select out 
the type we want and it will be almost entirely pure; that is, 
nearly all the plants selected will reproduce true to type as far as 
the characters wanted are concerned. Then selecting individual 
plants of the type wanted we can quickly get plants that are ho- 
mozygote with reference to practically all their characters by plant- 
ing the seed of each plant separately and observing which of them 
do reproduce true to type. 

These fixed forms which occur in the progeny of hybrids are 
sometimes mistakenly called ''mutations." They are in no sense 
mutations of the sort comprehended by that much misused term 
as it is at present understood. They are simply recombinations of 
characters which, before the hybridization occurred, existed in 
different combinations. 

After these fixed forms are obtained the same laws apply to their 
selection as have already been described under the effect of selection 
on close-fertilized forms. Generally speaking, we can not modify 
165 , 



52 APPLICATION OF PKINCIPLES OF HEREDITY TO BREEDING. 



them by selection, but selection may be valuable as a means of holding 
them up to a high standard. 

Reverting again to the hybrid produced by crossing long-headed 
winter wheat with a club-headed spring wheat we shall now con- 
sider the effects produced, first, by mass selection and, second, by 
individual selection in the progeny of such a hybrid. Figure 5 
shows the result of mass selection for the winter club type. Since 
both the winter character and the club character are dominant in 
this cross, four of the nine types occurring in the second generation 
will appear to be winter club wheats. These are the upper four 
types of figure 5. The other five can be distinguished at once, 
because they will either show the long-head character or the spring 

character. To deter- 
mine whether or not 
a wheat is winter or 
spring in character it 
should be planted in 
the spring. If it makes 
a crop the same season 
it is a spring wheat. 
If it waits until the next 
season before it heads 
out it is a winter wheat. 
The four types in the 
second generation 
which appear to be win- 
ter club wheats are 
those having the con- 
stitution WWCC, 
WWCc, ^YwCC, and 
WwCc, in which W 
stands for the winter 
character, w for the spring character, C for the club character, and c 
for the long-head character. The first of these four types is already 
fixed and will reproduce itself faithfully. The others are heterozygote 
with reference to one or both character pairs, and will consequently 
the next year produce some progeny which will be either spring or 
long-headed wheat, or both. Suppose, now, that in the second gen- 
eration we discard everything except these four types. Figure 5 shows 
what the result will be. The diagram of figure 5 is easily interpreted 
if we understand that the space between the top horizontal line and 
the uppermost curved line represents type WWCC, the space between 
the two uppermost curves represents type WWCc, and so on. It is 
seen that in the third generation, type WWCC has increased greatly in 



Types. 

jmcc 

WWCc 
WwCC 

WwCc 

[WWcc 
I Wwcc 
wwCC 
wwCc 
wwcc 




2 3 4 5 

fenerations. 



Fig. 5.— Graphic illustration of ten generations of a hybrid in a self- 
fertilized species selected for type WWCC. The letters and spaces 
between curves liave the same significance as in figure 4. 



HYBRIDIZATION AND SELECTION. 



53 



proportion. Type WWCc about holds its own until the fourth gen- 
eration, after which it decreases. The same is true of type WwCC, 
but type WwCc decreases rapidly from the second generation on- 
ward. At the end of ten generations practically the whole popula- 
tion is of type WWCO, which is pure winter club wheat, with a 
very small admixture of other types. This shows the effect of mass 
selection after hybridization in the case of self -fertilized crops. The 
result is a much more rapid approach to the one type selected than 
occurs in the corre- 
sponding case with 
cross-fertilized crops as 
shown in figure 3. 

The effect of indi- 
vidual selection, i. e., 
selection in which the 
seed of the individuals 
selected is kept sepa- 
rate, is shown for self- 
fertilized crops in fig- 
ure 6. In this figure, 
as in the preceding, the 
space beginning oppo- 
site each type symbol 
shows the proportion 
of that type from gen- 
eration to generation. 
In this method of selec- 
tion we save each sec- 
ond-generation individual which appears to be of type WWCC. This 
includes all plants of types WWCC, WWCc, WwCC, Siud WwCc. The 
seed of each plant is kept separate. In the next generation we save 
seed only from those rows or plots in which there has been no splitting 
up. This gives us at once the type WWCC in pure and fixed form. 
This is shown in figure 6, where the space representing type WWCC 
occupies the whole diagram beyond the third generation. 

CROSS-FERTILIZED SPECIES. 

The problem of utilizing hybridization in the production of new 
forms in cross-fertilized species which are propagated from seed is 
somewhat complicated by the fact that the individuals to be used in 
crossing may themselves be heterozygote for many characters. Di- 
versity in such species differs from that in self-fertilized species in 
this respect: In the latter we usually have in homozygote form all 
the combinations possible of the characters found in the group, 

165 




jmcc 

WwCc 
[WWcc 

\wwCC 
\wwCc 

^^^^ Z 3 4 5 6 7 8 9 .0 

Generations. 

Fig. 6.— Graphic illustration of the effect ol individual selection in 
a self-fertilized species on progeny of the hybrid Ww Cc. After 
the third generation the race is pure for the selected characters 
(both dominant in this case). The letters and spaces between 
curves have the same significance as in figure 4. 



54 APPLICATION OF PEINCIPLES OF HEEEDITY TO BREEDING. 

while in cross-fertilized species we have the same combinations, but 
not in homozygote form. In self-fertilized species, when a desired 
combination is not at hand, we can easily produce it and get it in 
fixed form. In cross-fertilized species, if the desired combination is 
not at hand, we can get it by crossing, but can not be sure of keeping 
it unless it is a form that can be propagated vegetatively, such as 
berries, tree fruits, and potatoes. The best we can do in cross-fer- 
tilized species which are propagated from seed is to make a cross 
with a view to getting a certain desired combination of characters 
and then select the desired type until we get a fairly constant strain 
of it. The results that follow such selection have already been 
explained under the head of Cross-fertilized species," page 36, and 
illustrated in figure 3. 

It has already been hinted that in cross-fertilized species we 
should not try for too great uniformity, as such uniformity in cross- 
fertilized species usually goes with weak development. If by cross- 
ing and subsequent selection we can get two forms, each of which is 
homozygote for the same desirable characters but heterozygote for 
other characters which are of no importance, and then plant them 
so that the two forms will cross freely, we shall probably have the 
nearest approach to the desired end attainable in such forms. 

MENDELIAN ANALYSIS OF HETEROZYGOTE RACES. 

When an individual which is heterozygote for a given pair of 
allelomorphs is self-pollinated it breaks up in the next generation 
into three forms, or types, two homozygote and one heterozygote. 
Let our pair of allelomorphs be represented by Aa. With self- 
pollination we have in the next generation one-fourth AA, one-half 
Aa, and one-fourth aa. That is, one-fourth of the seed produced 
is homozygote for A, one-fourth for a, and one-half of it is heterozy- 
gote, Aa. Hence, in such species as apples, pears, strawberries, and 
cassava, which do not ordinarily reproduce true to seed because 
they are heterozygote, usually for many of their characters, if we 
self-pollinate them, in the next generation we get many forms that 
are homozygote for some characters. If the species will endure con- 
tinued self-pollination it is clear that we could in eight or ten gen- 
erations break up any variety of this kind into homozygote types 
that would reproduce true to seed. In fact, there are a good many 
varieties of some of the species just mentioned that do reproduce 
practically true from seed. The Royal Anne cherry of Oregon 
and Washington is a case in point. Many of its seedlings can hardly 
be distinguished from the original variety. Where it is possible to 
secure these homozygote forms by this process of breaking up into 

165 



HETEROZYGOTE CHARACTERS. 



55 



pure races, a process which we may call Mendelian analysis, such 
forms might be of great importance to the breeder. They might 
render possible the deliberate combination of highly desirable char- 
acteristics existing in different varieties. Some very interesting 
work of this kind has been done. Prof. S. M. Tracy, working under 
the direction of the writer, has thus obtained three races of cassava 
which reproduce practical^ true to seed. They are now being 
used in an attempt to propagate this crop from seed instead of from 
cuttings. If the attempt is successful, and it promises to be so, it 
will permit a considerable extension of cassava culture into latitudes 
in which it is not practicable to propagate cassava from cuttings, 
because of the difficulty of keeping the cuttings over winter. 

In Volume V of the Annual Reports of the American Breeders' 
Association, Dr. W. T. Macoun, of Canada, reports some very inter- 
esting facts regarding the seedlings of the Wealthy apple. There 
was probably some cross-pollination in this case, but the seedlings 
give very plain evidence of the parentage of this important variety 
of apple. This is an important and nearly virgin field of investiga- 
tion, and more work of this kind will be looked for with interest. 

HETEROZYGOTE CHARACTERS. 

In some crosses, or hybrids, characters appear in the first genera- 
tion of the cross that were not apparent in either parent. These 
characters may belong to either of two classes. First, they may be 
due to the heterozj^gote nature of the hybrid. Characters of this 
class can be taken advantage of by the breeder only when the plant 
concerned can be propagated vegetatively. In crops propagated 
from seed a character which is due to the heterozygote nature of the 
plant which bears it will appear in the next generation in only half 
the progeny. Such characters can not be fixed by selection; at least 
no one has as yet succeeded in doing so, and it is highly improbable 
that it can be done. 

The most common type of such characters is the well-known vege- 
tative vigor seen in many hybrids. It is not unusual in sorghum 
fields to see here and there a st,alk which is much larger and taller 
than the rest of the field. Investigation has shown that these plants 
are hybrids. The writer has noticed in his work with hybrid wheat 
that the first-generation hybrid is mu-ch more vigorous and stronger 
growing than either of the parents as a rule, though this vigor in 
hybrid wheats is not so marked as it is in sorghum. In corn it is 
especially marked. Dr. G. H. Shull by the close breeding of two 
varieties secured practically homozygote strains of them, which 
were then crossed. The yield of the hybrid was about five times 
that of the attenuated self-fertilized pure strains. It should be 

165 



56 APPLICATION OF PRINCIPLES OP HEREDITY TO BREEDING. 



remarked that the pure strains of corn, because of the close inbreed- 
ing, had become very weak yielders, while the hybrid yielded an 
exceedingly large crop. 

Dr. E. M. East, of the Connecticut Agricultural Experiment Sta- 
tion, in a similar manner produced on small plots at the experiment 
station yields of corn exceeding 200 bushels per acre from hybrid 
seed. Here is an important point for the corn breeder. 

A good deal of effort has been made to secure uniformity of the ears 
in corn by a system of rather close breeding. This method will give 
the desired uniformity, but the close breeding in a species naturally 
cross-fertilized is likely to lead to lessened yields. Is it not better 
to breed for excellence, taking care not to breed too closely, and let 
uniformity take care of itself ? A method of using two strains of corn 
in the breeding plot in order to secure hybrid seeds for the field crop 
has already been described. 

Increased vegetative vigor does not occur in all crosses. Before 
recommending the cross-breeding of any particular crop in order to 
secure increased vigor, the fact that in that crop the desired results 
will follow should be determined. 

The following are other cases of characters which occur only in 
heterozygote form. In the cross between Black Andalusian fowls 
and a variety of White Andalusians having black splashes on the 
feathers, here called White Andalusians for convenience, the hetero- 
zygote is blue. If these blue fowls be mated with each other one- 
fourth of their progeny is black, one-half blue, and one-fourth white. 
If Blue Andalusians are desired, 100 per cent of blues can be obtained 
only by mating blacks and whites. 

According to Professor Bateson, exactly the same phenomenon 
occurs in the Bredas, a breed of fowls found in Holland. 

A similar case occurs in the cross between a certain red primrose and 
a certain white primrose, reported by Professor Bateson. The hetero- 
zygote is purple and is known to the trade under the name of 
''Imperial Primrose." Fifteen years of persistent selection has 
failed to cause this primrose to come true to seed. Every year one- 
fourth of its seed produce plants having red flowers, one-half of the 
seed produce plants having purple flowers, while the remaining one- 
fourth have white flowers. 

Dr. G. H. Shull found a mottled character in the seed coat of cer- 
tain flrst-generation hybrid beans which proved to be a heterozygote 
character; that is, when it appeared it was always heterozygote 
and would then reappear in only half of the plants whose seeds 
were mottled. Prof. R. A. Emerson, of the University of Nebraska, 
found the same character in beans, as did also Professor von Tscher- 
mak, of Vienna, and Mr. Locke, of Ceylon. 

1G5 



HETEROZYGOTE CEIAEACTEES. 



57 



Cases like those cited seem to be due to the presence of two charac- 
ters m different A'arieties wliich are not manifest in tliose varieties: 
but when brought together by hybridization they react on each other 
in some miknown manner so as to give rise to a new character. At 
the same time they form a Mendehan pair and separate again on the 
formation of gametes. Perhaps in such cases two clu-omosomes, 
wliich meet to form a bivalent in the reduction division, each tlu-ow 
off mto the cell a different chemical substance, and these two sub- 
stances, by reacting on each other j give rise to the new character. 
Wlien these two clu"omosomes are not together in the same cell the 
character does not appear. 

Most of such characters are probably reversions to lost characters. 
The fact that there are mottled races of beans like those produced by 
Shull and others, and Avliich reproduce true to seed, is in favor of this 
suggestion. In these mottled beans which reproduce true to type 
we may suppose that each of the chromosomes in question produces 
both of the chemical substances wliich we have supposed give rise to 
the character. If this is the case, then nonmottled varieties of three 
types could arise from a mottled variety by the loss on the part of 
the clu'omosonies producing the two necessary substances of the 
power of producmg one or the other or both of these substances. If 
in one variety of beans one of these substances and in another variety 
the other substance is missing, crossing the two varieties woidd cause 
the lost character to reappear. Tliis matter will be considered more 
fuUy when vre are considering the subject of latency of hereditary 
characters. 

Thus far we have considered only those new characters arising in 
crosses and which appear only m heterozygote form. There is a sec- 
ond class of characters arising in crosses that may be fixed by proper 
procedure. In worldng with gillyflowers Bateson and his coworkers 
foimd an interesting case of tliis Idnd, wliich ^\dll be considered more 
fully under the head of latency and need only be referred to briefly 
here. In crossing a cert am wliite variety vritli a cream-colored 
variety the progeny produced red flowers, and in the second genera- 
tion some individuals were obtained having red flowers and reproduc- 
ing true to seed. Evidently tliis result was due to the fact that two 
characters neither of which produced any effect when alone but when 
brought together gave rise to a visible character did not form a 
Mendelian pau and could consequently both be transmitted together. 

The reason why a character of this type can be fixed is seen in the 
following. We have assumed that the new character arises by the 
bringing together of two other characters that are not alleloniorphic 
to each other ; that is, do not form a pan' which must separate on the 
formation of gametes. If we call one of these characters A, its 

165 



58 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 

absence a, the other B, and its absence h, the first-generation hybrid 
is constituted thus, AaBh. This hybrid produces four types of 
gametes, namely, AB, Ab, aB, ab. An ovule of the type AB uniting 
with pollen of the type AB gives AABB, a homozygote strain in 
which the new character is fixed. While such cases as this are not 
common, they may occasionally represent important advances in 
breeding. It is therefore well for the breeder to understand them. 
Several such cases have been found. They also probably represent 
reversions to lost characters, at least in most cases. 

Characters may also appear in the second generation of a hybrid 
that were not apparent either in the first generation or in either of 
the parents. This is especially the case when a character is hypo- 
static in one of the original parents of the cross; that is, when it is 
covered up or hidden by some other character. A case in point is 
the appearance of brown beans in the second generation of the cross 
between black and white, reported by Shull. Here the brown is 
hypostatic to black, i. e., obscured or hidden by the black, in the 
black parent. Letting B represent black, h its absence, D brown, 
and d the absence of brown, the formulae for the black and white forms 
and the hybrid between them is — 

Black, BBDD. 
White, bbdd. 
Hybrid, BbDd. 

The gametes produced by this hybrid are BD, Bd, hD, and hd. 
The union of an ovule of the type hD with pollen of the same type 
gives hhDD, a brown type. 

Similar cases are known in animals. 

POSSIBILITY OF ENTIRELY NEW CHARACTERS. 

While most apparently new characters that arise in crossing are 
probably reversions to lost characters, it is easily conceivable that 
entirely new characters might arise in this manner. It seems prob- 
able that some cases of reversion are due to reaction between chem- 
ical substances, one of which is derived from one parent and the 
other from the other. These substances are probably produced in 
the cells of the respective pure strains before the cross; but they 
produce no effect because they are not both present in the same 
cells. It is conceivable that in some races there may have occurred 
evolutionary changes that result in considerable modification of the 
chemical contents of the cells but which produce no visible effect 
on external characters. In two related races these evolutionary 
changes may be quite different, and when we cross two strains 
that have been separated for some thousands of generations we 
may get, by reactions between substances that in the respective 

165 



EECIPEOCAL CROSSES EVOLUTIONARY CHANGES. 59 

pure races are of no effect, entirely new characters which thus seem 
to appear suddenly, but which in reality may have been thousands 
of years in developing. Characters arising in this way would appear 
only in heterozygotes if the two factors brought together happened 
to form a pair. But if they did not form a pair the new character 
would be capable of being fixed. 

Some phenomena have occurred, especially in crosses between 
distinct species, which probably belong in the class here considered, 
though not enough work has been done in this direction to make this 
entirely certain. 

RECIPROCAL CROSSES. 

If in one cross we use the pollen of race A on the stigmas of race B, 
while in another cross we use the pollen of race B on the stigmas 
of race A, these two crosses are said to be reciprocal to each other. 
Ordinarily such crosses give identical results. In his work with 
hybrid wheats at the Washington Agricultural Experiment Station 
the writer made reciprocal crosses in three instances, and in each 
case the results of the reciprocal crosses were identical. There are 
cases, however, where reciprocal crosses give different results. In 
some species the plant produces more than one kind of pollen and 
only one kind of ovule, as Correns found in Bryonia. He crossed 
a dioecious species with a monoecious species. When he used pollen 
of the dioecious species the hybrids were male and female in equal 
numbers, but when he used the pollen of the monoecious species 
the hybrids were all female. A number of other cases are known 
which may be explained on a somewhat similar basis. In a few 
cases differences have appeared in reciprocal crosses for which no 
explanation has been found. The plant breeder should make a care- 
ful record of any such cases coming under his observation, as they 
may lead to important advances in our knowledge of the principles 
of heredity. 

EVOLUTIONARY CHANGES AND THEIR RELATION TO PLANT 

BREEDING. 

While a great deal of study has been given to the general subject 
of evolution, actual knowledge of how and why evolutionary changes 
occur is very limited. The discussion this subject has received 
has been largely theoretical, and usually in support of some theory 
as to the manner in which such changes occur. 

We may perhaps distinguish two or more classes of evolutionary 
changes. A complex organism is provided with many hereditary 
characters — that is, characters which appear in successive generations. 
These characters may change in the manner of expression. For 
instance, a species having purple flowers may change with reference 

165 



60 APPLICATION OF PRINCIPLES OF IIEKEDITY TO BREEDING. 

to the shade of coloring, or a variety may change in size, and so on. 
Again, a character may become latent, possibly lost entirely. Thus, 
a purple-flowered species by the loss or latency of a factor for purple, 
may become red. Again, a red flower might become purple by the 
revival of the latent factor for purple. 

It is probably safe to say that most evolutionary changes are of 
the classes mentioned in the preceding paragraph. Take, for instance, 
the color of wfld mammals. Nearly all mammals, so far as they have 
been studied, have the same factors for color. The differences in the 
colors of the different species have come about simply by modiflca- 
tions in these same factors. Yet, some evolutionary changes result 
in the development of new characters. Beards on grasses must 
at one time have been new organs. But changes of this kind are 
comparatively rare, and occur so seldom that we can take little 
cognizance of them in practical breeding work. 

As stated before, we do not know the cause of these changes. 
One school of biologists maintains that evolutionary changes are 
slow and gradual, another that they take place by instantaneous 
steps, which may be large or small — that is, that they are 'discon- 
tinuous." We are more interested here in the amount of change 
that may occur in a given time than in the manner in which such 
changes take place. The important point is that when evolutionary 
changes do occur they are usually permanent changes, and the new 
forms resulting are subject to the laws of selection and hybridization 
which have already been outlined. That these permanent changes 
do occur can not be questioned. That in general they are merely 
changes in hereditary characters already present is equaUy certain. 
Doctor Nilsson in his work with the cereals at Svalof has many times 
taken an unselected lot of seed from a standard variety of field 
grain and found it in the main to consist of a large number of fijced 
types differing from each other in various ways. When the same 
character is studied throughout the numerous strains that occur in 
a field it is found to present nearly every possible gradation in 
different strains, but generally speaking in each of the strains the 
gradation found is fixed. 

Jennings, in his study of Paramecium, found in wild cultures almost 
an. indefinite number of strains, each differing permanently in size, 
and these differences undoubtedly are due to permanent changes 
of hereditary characters, as in the case of Nilsson's cereals. Jennings's 
investigations indicate that there might be found in Paramecium 
almost every gradation in size, but that the size of each particular 
strain is fijced. 

The principal relation of these changes to the work of the plant 
breeder lies in the fact that a crop as grown under field conditions will 

165 



PLACE EFFECT, 



61 



usually consist of a large number of strains which differ from each 
other on account of evolutionary changes which have occurred in 
the past, and the breeder can by selection secure the strains which are 
of greatest value. 

A few instances are known in which important changes have been 
brought about by persistent selection. De Vries, by continued 
selection from a 13-rayed strain of Chrysanthemum segetum, was 
finally able to produce a double-flowered variety. Burbank found 
a single specimen of California poppy which had a scarlet line on 
one of its petals. By persistent selection from this plant he was 
able to get a scarlet poppy. Cases of this kind are not understood, 
yet they are important from the standpoint of the breeder. They 
show that we have much yet to learn in this important field. It 
may be stated that scarlet-flowered California poppies occasionally 
occur in nature. This fact shows that at least part of the species 
has the scarlet color factor. It is probable, therefore, that Burbank 
started with a plant having this factor, and by selection merely 
eliminated other color factors. 

PLACE EFFECT. 

It is a well-known fact that when a crop is moved to a locality 
which is radically different from that in vhich it was previously 
grown it sometimes behaves in a surprising manner, presenting 
unlooked-for variations. This subject has not been studied nearly as 
much as it ought to be. We really do not know the behavior of the 
variations which occur under such conditions, because so little atten- 
tion has been given to them. 

Take, for instance, the case of the tomato in southern climates. 
Some of the standard varieties of tomatoes present very peculiar 
modifications when seed grown in the North is planted in the far 
South. The first year the fruit is normal and resembles the fruit 
that would have been produced in the North; but if seed of the south- 
ern-grown fruit be saved and planted, the next year the fruit is of a 
very different character and remains so from generation to genera- 
tion under the new conditions. The writer is informed by Mr. 
W. W. Tracy, sr., of this Bureau, that when the seed of these trans- 
formed tomatoes is taken back to the North and planted, while the 
first year it grows the southern type of tomato, the next year it 
reverts to its northern form. Cases of this kind are well worth more 
attention than they have received. 

It is to be noted that in the case of the tomato just referred to the 
same change occurs in all the individuals. In this connection, 
some work recently done by Dr. Albert Mann, of this Bureau, is of 
special interest. Although the results are as yet unpublished Doctor 
Mann kindly permits me to refer to them, 

165 



62 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 

Three years ago he obtained from Doctor Nilsson, of Svalof , a num- 
ber of the pedigreed or pure-hne strains of barley grown at that sta- 
tion. Seed of five or more of these were sent to thirty-eight locahties 
throughout the country in 1907, representing nearly two hundred 
tests. In all cases more or less transformation occurred in each of 
the strains under investigation. Generally speaking, every indi- 
vidual of a given strain went through identically the same trans- 
formation in the same locality. The results obtained the second 
year on some thirty locations indicate that the changes made by 
these plants are permanent as long as they are grown under the new 
conditions. 

In several instances it was found that a given strain did not behave 
alike, part undergoing one modification and part another. A care- 
ful study of one of these cases revealed the fact that one end of the 
plot w^as on sandy soil and the other on loam and that all the plants 
at the same end had suffered the same change. 

Doctor Mann has called my attention to the important fact that 
these changes suffered by pure lines when taken to a radically differ- 
ent environment from that to which they had been accustomed 
seem in no way to be adaptive changes. They are apparently not 
adjustments to the new conditions, but are changes caused by the 
new conditions. Apparently, they may be advantageous or dis- 
advantageous to the plant under its new surroundings. 

It is easily seen that by studying this question with types of plants 
from which all other kinds of variation have been eliminated, results of 
fundamental importance may be obtained. The conclusions, which 
are at least indicated by the very meager data at hand, are that these 
new-place effects produce similar results on similar individuals, that 
they are permanent under the changed conditions, and that they 
are fortuitous in character. It is by no means established, however, 
that these conclusions are general. This is evidently an important 
and nearly virgin field for investigation. 

NON-MENDELIAN CHARACTERS. 

The only case known to the writer of anon-Mendelian character which 
has been clearly made out and for which the method of inheritance has 
been determined is one recently published by Dr. Erwin Baur, of Berlin, 
and which relates to the method of inheritance of the white margin 
of certain leaves.^ The white-margined plants produce only pollen 
and ovules carrying the white tissue character. But when these 
plants are crossed with ordinary green plants the new individual 
thus formed is capable of producing both kinds of tissue. It would 

o Since the above was written the publication of Castle's monograph on inheritance 
in the rabbit has been made by the Carnegie Institution. Had this publication 
appeared earlier it would have received extended notice in these pages. 
165 



NON-MENDELIAN CHARACTERS. 



63 



appear that in the cross-fertilized ovule, part of the cytoplasm of 
the cell carries the tendency to develop green tissue and part the 
tendency to develop white tissue. If at any cell division one of the 
daughter cells should happen to receive only cytoplasm of a certain 
kind then the tissue descended from that daughter cell will be either 
pure white or pure green, as the case may be. Plants originating 
from this cross between white and green are thus called mosaics. 
A leaf or bud originating on the line of contact between the tissues 
will be white on one side and green on the other. If it originates 
wholly from white tissue it will be pure white; if wholly from green 
tissue it will be pure green. Occasionally, however, the white tissue 
on the stem of such plants may extend as a thin surface layer over 
the green tissue. A bud coming through such a layer will be com- 
posed of green tissue within and a thin layer of white tissue without, 
and this bud gives rise, by division and propagation, to a new white- 
margined plant. 

Another case which should be mentioned here is that of the in- 
heritance of ear length in rabbits studied by Prof. W. E. Castle, of 
Harvard. The cross between long-eared and short-eared rabbits 
had ears intermediate in length, and their progeny were like the 
hybrid in this respect. In this case the mechanism of inheritance 
is not clear; and it is barely possible that it is simply a very complex 
case of Mendelian inheritance. 

In a great many crosses between very distinct species we do not 
get strictly Mendelian phenomena and we do not know exactly 
why. It is highly probable, however, in the writer's opinion, that 
the reason is to be sought in the following. Most of the organs and 
parts of an organism are developed as the result of the interaction 
of a good many factors which are Mendelian in nature. For in- 
stance, the development of horn tissue in cattle may be the result of 
the interaction of three or four or even more chemical substances 
arising from different organs in the cell. Now, if in two races of 
cattle v/e find a difference in only one of these chemical substances, 
then the differences between the two races would behave as a simple 
Mendelian character; but if there were differences in all the sub- 
stances concerned we should have an exceedingly complex case of 
Mendelian inheritance, the unraveling of which would require such 
large numbers of progeny from hybrids that it would be practically 
impossible to determine the nature of the inheritance in the case. 
The writer is of the opinion that the lack of simple Mendelian char- 
acters in species hybrids is partly of the nature here outlined. In 
some species crosses apparently wide departures from Mendelian 
principles occur. Take, for instance, the cross made by Burbank, 
resulting in the Primus berry. Here there was wide diversity in the 
first-generation hybrids. The one first-generation individual from 
81599°— Bui. 165—11 5 



64 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 

which the Primus berry is descended was unhke any of the others. 
But this individual has reproduced true to seed from the beginning. 

Kosenberg has shown that in a similar case in the genus Hieracium 
the first-generation hybrids, which are not alike, have different 
numbers of chromosomes. Presumably, in such cases, the chromo- 
somes of the two species crossed differ so much in habit that they 
can not function together properly, and some of them are lost in the 
early cell divisions of the young hybrid. In some individuals, one 
chromosome is lost, in others other chromosomes are lost; so that, 
if the supposition here made is true, the young first-generation 
hybrids do not have the same complement of hereditary characters, 
which would account for their not being alike. 

The fact that these hybrids do not split up in Mendelian fashion 
in the second and later generations suggests that the corresponding 
chromosomes in the two sets of chromosomes brought together in 
these wide crosses are so unlike that they are not drawn together 
to form bivalents in the reduction division. That is, the reduction 
division does not occur in the mother cells which produce ovules 
and pollen. If this should prove to be true, then either of two alter- 
native courses of events would give hybrids which reproduce true 
to seed without Mendelian splitting. 

(1) Seed may be produced parthenogenetically, without the in- 
tervention of a reduction division and subsequent fertilization, and 
this asexual production of seed may continue from generation to 
generation. 

(2) Without a reduction division in the first-generation hybrid, 
a pollen nucleus may unite with an ovule nucleus, thus giving a 
nucleus having two sets of chromosomes like the one possessed by 
the first-generation hybrid. The set of chromosomes in the first- 
generation hybrid is presumably composed of chromosomes part of 
which are from one species and part from the other. The double 
nucleus thus formed will have two sets of chromosomes, every one 
in one set having an exact duplicate in the other. After this, that 
is, in the second and later generations, reproduction would take place 
in the usual manner, without any Mendelian splitting, for the two 
halves of each bivalent formed in the reduction division would be 
exactly alike. 

It would probably be possible, by cytological study of these con- 
stant hybrids, to determine whether the absence of Mendelian split- 
ting is due to either of the causes above suggested. 

MUTATIONS. 

The term ''mutation" has been used and is now used in so many 
senses that a great deal of confusion has arisen in consequence. 
Prof. Hugo De Vries, of Holland, after investigating some hundreds 

165 



LATENCY. 



65 



of species of plants, found one species {Oenothera lamarclciana) 
which occasionally produced offspring that were unlike the parent. 
Some of the new individuals were capable of reproduction true to 
type. These forms he called mutations. The work of Dr. R. R. 
Gates, of the University of Chicago, who is making a careful study 
of the chromosome behavior in these mutants, indicates strongly 
that these mutations are due to accidents occurring in the reduction 
division by which chromosomes are either lost or gained or ex- 
changed, so that some of the daughter cells are provided with a set 
of chromosomes differing from that of the parent species. A good 
deal of work must yet be done before this matter is settled, and 
until we know more about it we can not assign mutations of this 
class to their proper place in heredity and in evolutionary progress. 

The term '^mutation" has also been applied to any permanent 
evolutionary change of whatever magnitude or whatever its cause. 
It is hardly probable that all evolutionary changes are due to acci- 
dents in cell division. It would seem rather that most of such 
changes are due to permanent changes in whatever material is 
responsible for the development of hereditary characters, and it is 
probable that we shall ultimately have to make a distinction between 
these two types of so-called mutations. 

LATENCY. 

Characters sometimes fail to develop, although present. The 
reason for such failure may fall into any one of several categories. 
One of the most important recent papers dealing with this phase of 
the subject is that of Dr. G. H. Shull in the American Naturalist, 
Volume XLII, July, 1908. The following classification of the differ- 
ent types of latency follows, in the main, Doctor Shull's paper, 
departures therefrom being noted in the text. 

I. LATENCY DUE TO SEPARATION. 

Bateson and his coworkers crossed a cream-colored strain of 
gillyflowers with a white strain and secured a hybrid which had red 
flowers. This is explained by assuming that in the cross two char- 
acters are brought together which when separated are incapable 
of producing red flower color, but which when together give rise to 
this color. Many other breeders have found similar instances, 
and these characters, which when alone produce no visible effect 
but when in the presence of other characters give rise to visible 
manifestations, have been called ' ' cryptomeres " by Prof. E. von 
Tschermak, a term which is derived from the Greek and which 
etymologically means ''hidden parts." Both von Tschermak and 
Bateson have shown that purple color in gillyflowers is due to three 
such cr}^tomeres. Two of these without the third give rise to the 

165 



66 APPLICATION OF PEINCIPLES OF HEREDITY TO BREEDING. 



red color. When the thn*d is present with the first two purple arises. 
There are other cryptomeres present in gillyflowers which modify 
these colors, but the numerical relations in their transmission have 
not yet been fully made out. 

Shull found a cryptomeric character in beans in a cross between 
certain brownish-seeded beans and white-seeded beans. It appears 
that the white variety carried a cryptomere which when present with 
the factor which gives rise to the brown color converts the brown 
into black, thus giving in the hybrid a character which was apparently 
absent in both of its parents. 

In this case we may represent the formula of the two parents as 
follows: 

White parent = BBpp. 
Brown parent = hhPP. 

Here B stands for the cryptomere w^hich converts the brown 
color into black (and which is latent in the absence of P) and P for 
the producer of the brown pigment. The hybrid would have the 
formula BhPp, The second generation of this hybrid would be as 
follows : 



B. P. pb. 

1 BBPP 1 

2 BBPp 2 

1 BBpp 1 

2 BhPP 2 
4 BhPp 4 

2 Bbpp 2 



B. P. pb. 

1 bbPP 1 

2 bbPp 2 

1 bbpp 1 



16 



The first column in the above table gives the nine types in the sec- 
ond generation of the hybrid , and the figures at the left of the formula 
show the relative frequency of the types. Thus type BBPP consti- 
tutes one-sixteenth of the second generation, type BBPp two-six- 
teenths, and so on. Since B and P are both present in types BBPP, 
BBPp, BhPP, and BhPp, these four types, constituting together 
nine-sixteenths of the second generation, will all be black. Types 
hhPP and hhPp are brown, while types BBpp, Bhpp, and hhpp 
will all be white, although two of them have the factor B. The 
factor B has no effect in the absence of P. We thus have in the 
second generation of this hybrid 9 blacks, to 3 browns, to 4 whites. 

A more complex case of this kind is that of the purple gillyflowers 
already mentioned. In this case we have to deal with the following 
factors : 

C=one factor of red. 
P=other factor of red. 
P=the factor for purple. 

Of these factors C and R are cryptomeric to each other; that is, 
neither of them produces a visible effect except in the presence of 
the other. P is cryptomeric to both C and R. The factor P was 



LATENCY. 



67 



first discovered by crossing a cream-colored variety carrying the 
factor C and a white variety carrying the factors R and P. The 
first-generation hybrid was therefore CcRrP]). The second genera- 
tion of this cross gave the following: 







P. 


P. 


(or crG&m) . 




1 


CCRRPP 


1 






2 CcrrPP 


2 


CCRRPp 


2 






4 CcrrPp 


1 


CCRRpj) 




1 




2 Ccrrpp 


2 


CCRrPP 


2 






1 ccRRPP 


4 


CCRrPp 


4 






2 ccRRPp 


2 


CCRrpp 




2 




1 ccRRpp 


1 


CCrrPP 






1 


2 ccRrPP 


2 


CCrrPp 






2 


4 ccRrPp 


1 


CCrrpp 






1 


2 ccRrpp 


2 


CcRRPP 


2 






1 ccrrPP 


4 


CcRRPp 


4 






2 ccrrPp 


2 


CcRRpp 




2 




1 ccrrpp 


4 
8 


CcRrPP 
CcRrPp 


4 
8 






64 


4 


CcRrpp 




4 







(or cream). 
2 
4 
2 
1 
2 
1 
2 
4 
2 
1 
2 
1 



27 



28 



This gives in the second generation 27 purples, 9 reds, and 28 whites 
or creams. 

The above illustrations will give the reader an idea of the behavior 
of crj^ptomeric characters. These characters are not infrequent, 
and in the early days when such cases arose they were frequently 
reported as exceptions to Mendel's law." Their inheritance is now 
well understood. 



II. — LATENCY DUE TO DOMINANCE OF ABSENCE OVER PRESENCE, 

Latency due to dominance of absence over presence was not 
separately considered by Shull in the paper referred to. Generally 
speaking, the presence of a character is dominant to its absence, 
but there are some exceptions. Two of the best known relate to 
horns in cattle and beards in w^heat. We do not know exactly why 
these characters do not develop when they are in the heterozygote 
state. It is possible that in both these cases the absence of horns 
or beards may be due to the presence of some inhibiting factor, in 
which case the present category of latency would be a simple case 
of dominance of presence over absence. It seems probable, however, 
that these characters are simply unable to develop unless represented 
by two allelomorphs. The reasons for this assumption are rather 
too abstruse to be given here, for to make them clear would require 
a great deal of space, and they are also more or less speculative at 
the present time.^ 



a See article by Shull in American Naturalist for July, 1909. 



68 APPLICATION" OF PRINCIPLES OF HEREDITY TO BREEDING. 



III. LATENCY DUE TO HOMOZYGOSIS. 

This group of facts might perhaps be better described as '^patency 
due to heterozygosis." It includes those cases where a character 
is patent only in heterozygotes. The following discussion differs 
in some details from that given by Shull. 

In Shull's bean crosses, mottled first-generation hybrids occurred 
between varieties neither of which was mottled, and subsequent 
investigation showed that the mottling only occurred in heterozygotes. 
Tschermak and Locke have both reported similar cases; also Professor 
Emerson, of the University of Nebraska. In all these cases the 
mottled beans produce progeny half of which is mottled and the 
other half not, thus giving a departure from the usual 3:1 ratio found 
in ordinary Mendelian characters. 

The following explanation of all the above cases is here offered. 
The explanation vAll be given for mottled character, from which the 
explanation of the other cases may be easily inferred. The fact 
that in beans there are mottled varieties which breed true and 
w^hich when crossed with the new type of mottled beans give ordinary 
Mendelian phenomena greatly strengthens the hypothesis here stated. 
Let us suppose that originally the mottling was due to two correlated 
characters; that is, to two characters which are always transmitted 
together. We may represent the determiner for this double character 
as M". The formula for those strains of mottled beans which re- 
produce true to type would then be M M . If we suppose that in 
some strains of these mottled beans the character a becomes latent 
or is lost, while in other strains the character c does likewise, while 
in still others both a and c become latent, we get three types of 
nonmottled beans, the formulae for which are M^M^', MgMc, and MM. 
A cross between the first and second of these nonmottled types would 
give mottling of the character found by Shull. This cross would have 
the formula M^^M^. In the next generation this would split up into one- 
fourth M«M«, one-half M^M^, and one-fourth M^M^, in which only the 
heterozygotes would be mottled, for it is only in them that we have 
both factors of the mottling. Either of the three types of nonmottled 
beans crossed with permanently mottled beans would give the ordi- 
nary Mendelian behavior of the mottled character in which in the 
second generation we should have three mottled to one nonmottled. 

This same explanation is in accord with the facts in the case of the 
blue color in Andalusian fowls and the purple color in Imperial prim- 
roses. In the case of the blue Andalusians the blue may not be a rever 
sion to a lost character, but may be, in a sense, a new character; but on 
the above explanation its presence in heterozygotes is assumed to 
be explicable on exactly the same basis as the presence of mottling 
in those beans in which it occurs only in heterozygotes. 

165 



LATENCY. 



69 



IV. LATENCY DUE TO HYPOSTASIS (MASKING). 

Sometimes a character may be hidden by the presence of another 
character which simply obscures it. Thus, in the hair of most species 
of mammals there are both yellow and black pigments, each of which 
may be visible because of a certain other factor which causes them 
to develop more or less in different regions of the same hair. But in 
the absence of this controlling factor the black and yellow pigments 
develop in the same regions of the hair and the black thus obscures 
the yellow. Prof. T. H. Morgan found a case in the cross between the 
black rat and the Alexandrian rat, in which the black color was 
dominant over the gray color of the Alexandrian rat, presumably 
because the black color hid the other color characters. 

Characters which are thus obscured by the presence of another 
character are said to be latent by hypostasis — that is, they are 
hypostatic to the obscuring character, while the latter is said to be 
epistatic to the obscured character. These convenient terms were 
suggested by Professor Bateson. In one of his crosses between 
black beans and yellow beans ShuU obtained some seal-brown beans, 
and inferred that the seal-brown had been present in the black 
beans, but was there obscured by hypostasis. 

V. LATENCY DUE TO INHIBITION. 

The category of latency due to inhibition is much like the last and 
was included by Shull, perhaps properly, wdth it. It seems possible, 
however, that it may deserve separate treatment. In the cases 
considered characters have been invisible simply because some other 
character present obscured them. There are cases, however, where 
the presence of one character seems to prevent the development of 
another character. For instance. Prof. V. L. Kellogg, of Leland 
Stanford Junior University, in crossing certain white and certain 
colored varieties of silkworms found the white to be dominant. Simi- 
lar phenomena have been found by Bateson and Davenport in 
poultry, and the writer, in cooperation with Mr. Q. I. Simpson, has 
found the same in swine. Here the presence of the white character 
seems to prevent the color from developing rather than simply to 
obscure the color. Perhaps we might not be justified in treating 
this case as anything else than hypostasis. At any rate, the behavior 
in inheritance is exactly as in the case of hypostasis, as far as the 
ratios of the various types are concerned. 

VI. LATENCY DUE TO FLUCTUATION. 

Some MendeUan characters are highly variable. Shull cites the 
case of certain leaf lobings which vary greatly under unfavorable 
conditions, and sometimes entirely fail to develop. When the 



70 APPLICATION OF PEINCIPLES OF HEREDITY TO BREEDING. 

plants m question were grown under favorable conditions, it was 
easy to demonstrate that leaf lobings are a good Mendelian char- 
acter, but under certain unfavorable conditions the lobing disap- 
pears, thus confusing the Mendelian results. Kellogg found a similar 
case in silkworms. Certain strains of silkworms which produce 
salmon-colored cocoons when crossed with certain white strains 
gave results which showed clearly that the salmon color is a Men- 
delian character. But in some crosses the salmon color became 
extremely variable, extending all the way from almost pure white 
to very deep salmon color, thus somewhat obscuring the numerical 
relations of the colors in the second generation of the hybrid. In 
some of Correns's work with variegated plants he found a similar 
character. The variegation, although a good Mendelian character, 
varied with true green leaves as one extreme, and in some crosses 
this fact made the number of green leaves in the second generation 
larger than theory called for. 

CORRELATION. 

A good many cases have been found where two characters wliich, 
so far as appearance goes, are not physiologically related to each 
other seem always to be transmitted together, and the breeder 
frequently gets irregular results because of this coupling or correla- 
tion of characters. For instance, Tschermak found in Chinese oats 
that hull-less seed is correlated with long, many-flowered spikelets, 
and . that these two characters were always transmitted together. 
In Price and Drinkard's work with tomatoes at the Virginia Agri- 
cultural Experiment Station they found what seems to be a case 
of this kind. In one of their crosses one of the parents had green 
foliage and two-celled fruit. The other parent had 3^ellowish green 
foliage and many-celled fruit. The hybrid was like the first parent. 
In the second generation of this hybrid all the plants having green 
foliage had two-celled fruits, and all those having yellowish green 
foliage had many-celled fruits, thus indicating that the many-celled 
condition of the fruit is transmitted with the yellowish green leaf 
character, at least in this case. Hedrick and Booth, in their work 
with tomato hybrids, found that dwarf stature was correlated with 
dark-green rugous leaves, while standard stature was similarly 
correlated with lighter green smooth leaves. 

Such cases sometimes give the breeder a good deal of difficulty. 
They are of special importance in relation to theories of heredity, 
and should always be carefully noted and reported. 

165 



INDEX. 



Page. 

Allelomorphism, definition 17 

Allelomorphs, characters and determiners of characters in plant breeding 17 

Andre\rs. ^Y. A., experiments in propagation of dahlias, and results 31 

Apple. Baldwin, tree heterozygote in character 47 

Wealthy, seedlings, study in Canada 55 

Apples, heterozygote species 54 

Bacteria, evolution 29 

Banana, plant vegetatiA-ely propagated, producing no seed but retaining vigor. . 25 

Barber, M. A., study of experimental evolution of yeasts and bacteria 29 

Barley, individual selection, experiments in Ontario 34, 35 

Mandscheuri, mass selection in Ontario, experiments and results 35 

self -fertilized, effect of selection 32-33 

mass selection, effect on yield 34 

study of effect of location 61-62 

Bateson, William, experiments in crossing gilhiiowers. and results 57, 65-66 

investigations of allelomorphism 17 

study of fowls and the Imperial primrose 56 

Bam'. Erwin, study of white-margined plants 62-63 

Beans, crossing, experiments and results 66-67, 68, 69 

hybrid, character 56-57, 58 

study 56 

hybrids, mottled, experiments and results 68 

Berry, Primus, crossing experiments and results 63 

Booth, studies of tomato hybrids , 70 

Breeding, plant, effect of location on occurrence of variations 61-62 

relation of evolutionary changes 59-61 

Bud sports. See Sports, bud. 

Burbank. Luther, experiments in crossing Primus berry 63 

with poppies in California 61 

California poppy, selection, results 61 

Canada, Wealthy, apple, reports of American Breeders' Association 55 

Carnation, short-lived variety. 25 

Carnegie Institution, experiments in crossing poultry 14 

Cassava, heterozygote species 54, 55 

races, study 55 

Castle, W. E., study of long-eared and short-eared rabbits 63 

Cattle, crossing, characteristics 63 

polled and horned varieties, results 7-8, 16 

Durham, experiments in crossing with Hereford cattle, table 18-20 

Hereford, experiments in crossing with Durham cattle, table , 18-20 

horns, development and causes 67 

165 71 



72 APPLICATION OF PEINCIPLES OF HEREDITY TO BREEDING. 



Page. 

Characters of plants, correlation 70 

fluctuating variations 2S-25 

heterozygote 55-58 

latency, causes 65-70 

new, possibility 58-59 

non-Mendel ian 62-64 

variations resulting from change of location 61-62 

Cherry, Royal Anne, homozygote species 54-55 

Chicago University, mutation studies 65 

Chickens, Barred Plymouth Rock, mating with Indian Game chickens^ results. 14-15 
Chromosomes, study of potential elements controlling sexual differences in 

plant breeding 11 

Chrysanthemum segetum, selection and results 61 

Collins, G. N., bulletin entitled "The Importance of Broad Breeding in Com," 

reference 44 

Colorado, potato-growing experiments 25 

Connecticut, corn-breeding methods, study 44 

Copenhagen, experiments in breeding beans and barley 24 

study of effect of seed selection 33 

Corn, breeding, broad, importance 44 

experiment station studies in Delaware 44 

cross-fertilized, effect of selection for high or low ears, table 37 

on oil and protein content , 36 

selection, results 37-46 

detasseled, planting, experiments and results 46 

hybrid, yield 55,56 

■ open-pollinated, breeding and planting methods 44-46 

seed selection, results 44 

white, planting alternately with yellow seed 45 

yellow, planting alternately with white seed 45 

Cornfield, experiments in fluctuating variations 23-25 

Correlation of characters of plants 70 

Corren, study of variegated plants 70 

Crops, self -fertilized, mass selection 33-34 

vegetatively propagated, hybridization and selection 46-47 

Crosses, reciprocal 59 

Crossing, artificial, selection without 26-46 

Dahlia, bud variations 31 

Dandelion, nonfertile, producing seeds and retaining vigor 25 

Davenport, C. B., studies in poultry crossing 14 

Delaware, corn breeding, experiment station studies 41 

De Vries, Hugo, experiments with Chrysanthemum segetum 61 

study of plant mutations 64-65 

Dodge, L. G., experiments in seed selection of potatoes 31-32 

Dominance, principle of heredity in plant breeding, examples 7-8 

Drinkard, experiments in crossing tomatoes 70 

East, E. M., methods of corn breeding 44, 57 

study of fluctuating variations in potatoes 26 

Emerson, R. A., study of hybrid beans . 56, 68 

England, Cambridge, studies in allelomorphism 17 

Evolution, plant-breeding changes and their causes 59-61 

Fluctuation, a cause of latency of characters of plants 69-70 

165 



INDEX. 73 

Page. 

Fowls, Andalusian, black, white, and blue, crossing experiments 56 

color, development and causes 68 

Bredas, crossing experiments 56 

crossing, results 14—15 

Gamete, study of reproductive cell in plant breeding 17, 18, 19, 57-58 

Gardner, F. B., breeding of corn, recommendations 44-45 

Gates, R. R., studies of plant mutation 65 

Germany, Breslau, experiment station studies of rye 37 

Gillyflowers, crossing experiments and results 57, 65-66 

Goodale, H. D., study of poultry crossing 14-15 

Grantham, A. E., studies in seed-corn selection 44 

Grubb, E. H., maintenance of variety of Peachblow potato in Colorado 25 

Han el, Elise, study of evolution of hydra 29 

Hartley, Charles P., breeding of corn 44 

Hays, Willet M., experiments in plant selection in Minnesota 35-36 

Hedrick, studies of tomato hybrids 70 

Heterozygote characters, study 55-58 

Hieracium, first-generation hybrid, crossing experiments and results 64 

Homozygosis, a cause of latency of characters of plants 68 

Hybridization and selection, cross-fertilized species 53-54 

self -fertilized species 47-53 

planting of corn 45-48 

studies of organism differences in plant breeding 11-12 

vegetatively propagated crops, advantages and results 46-47 

Hybrids, hereditary characters 64 

Hydra, evolution 29 

Hypostasis, a cause of latency of characters of plants 69 

Illinois Agricultural Experiment Station, studies in potato growing 26 

selection of cross-fertilized 

species 36 

wheat growing 47-48 

Indian Game chickens, mating with Barred Plymouth Rock chickens, results. . 14-15 

Inhibition, a cause of latency of characters of plants 69 

Introduction to bulletin 7 

Jennings, H. S., study on unicellular animal Paramecium, table. . .•. 26-29, 60 

Johannsen, W., experiments in selection of self -fertilized plants 33 

investigations of fluctuating variations of wheat and barley 24—25 

Kellogg, V. L., experiments in crossing silkworms ■ 69-70 

Latency of characters of plants, experiments and results 65-70 

Lawrence, C. W., investigations with hybrid wheats 49 

Leaf, lobing, study in development and causes 69-70 

Locke, study of hybrid beans 56, 68 

London, Genetic Conference, study of effect of seed selection 33 

Macoun, W. T., study of apple seedlings in Canada 55 

Maine, studies of selection in plant breeding 30 

Mammals, color of hair, development and causes 69 

Mann, Albert, study of effect of location on barley 61-62 

Mendel, Gregor, discoverer of principles of dominance and recessiveness in 

plant breeding 7 

Mendelian analysis of heterozygote races of plants 54-55 

Morgan, T. H., study in crossing rats 69 

Morrow, G. E-, breeding of corn, recommendations 44-45 

Mosaics resulting from crossing plants 63 

165 



74 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 



Page. 

Mutations, plant, investigations and results 64—65 

Nebraska, University, study of characters of beans 56 

New England, experiments in growing potatoes 31 

Nilsson, experiments in producing new cereals 49 

selection of self -fertilized plants 24-25, 32-3,3 

studies on evolutionary changes in plant breeding 60 

study of effect of location on barley 61-62 

Oats, Chinese, crossing experiments and results 70 

self -fertilized, effect of selection 32-33 

mass selection, effect on yield 34-35 

Oenothera lamarckiana, plant-mutation studies and results 64-65 

Ontario, Guelph, studies in selection of self -fertilized crops 34 

Oregon, cherries, growing seedlings : 54-55 

Ovule, study of fertilization germ of hybrid plants 9-17 

Paramecium, unicellular animal, summary of study on experimental evolution. 26-29 

Pea, garden, crossing two varieties, result 7, 8 

Pearl, Raymond, study of close-fertilized species of plants 30 

Pears, heterozygote species 54 

Place effect, variations in characters 61-62 

Plant breeding. See Breeding, plant. 

Plants, centgener power, result of environmental conditions 35 

characters, correlation 70 

heterozygote 55-58 

latency, causes '. 65-70 

new, possibility 58-59 

non-Mendelian 62-64 

cross-fertilized, individual selection, effect 36-46 

problem of improvement 43-44 

selection and results 53-54 

experimental evolution, studies 26-32 

mutations, investigations and results 64-65 

races, heterozygote, Mendelian analysis 54-55 

vegetatively propagated and self -fertilized 25-26 

self -fertilized, effect of selection 32-36 

individual selection* 35-36 

mass selection 33-34 

methods of selection 33-36 

variegated, study 70 

vegetatively propagated, fluctuating variations due to environment 26-32 

tendency to run out 25-26 

white-margined, study 62-63 

Plymouth Eock, Barred, chickens, mating with Indian Game chickens, results. 14-15 

Pollen, dioecious species of plants, use and results 59 

monoecious species of plants, use and results 59 

study of fertilization germ of hybrid plants 9-17 

Poppy, California, selection and results 61 

Potato, experiments in seed selection 31-32 

Peachblow, long-lived variety 25 

Potatoes, bud sports, characters exhibited 35 

hereditary characteristics and seed selection 30, 31-32 

seedling, lack of hybridization 46 

self -fertilized, mass selection, effect on yield 34 

165 



INDEX. 7 5 

Page. 

Poultry, crossing 14-15, 69 

Price, experiments in crossing tomatoes 70 

Primrose, hybrid, breeding, progeny produced and causes 9-17 

hybridizing experiments in England, results 9 

Imperial, color, development and causes 68 

Propagation, vegetative, fluctuation, effect of environment 26-32 

Rabbits, long-eared and short-eared, cross, study 63 

Races of plants, heterozygote, Mendelian analysis 54—55 

Rats, crossing experiments, development and causes 69 

Recessiveness, principle of heredity in plant breeding, examples 7-8 

Recombination, laws and results 18-2S 

Rosenberg, study of Hieracium genus 64 

Riimker, von, experiments on cross-fertilization of rye 37 

Rye, cross-fertilization, experiments 37 

Segregation, natural laws and results 8-17 

Selection and hybridization, cross-fertilized species 53-54: 

self -fertilized species 47-53 

vegetatively propagated crops 46-47 

effect on cross-fertilized species 36-46 

self-fertilized species 32-36 

individual, self-fertilized plants 35-36 

mass, self-fertilized plants 33-34 

without artificial crossing 26-46 

Separation, a cause of latency of characters in plants 65-66 

Shull, G. H., experiments with hybrid corn, results 55-58 

method of breeding open-pollinated corn 44, 46, 55-56 

paper on causes of latency of characters of plants 65-70 

study of hybrid beans 56-57 

Silkworm, crossing experiments, development and causes 69, 70 

Simpson, Q. I., studies in crossing swine 69 

Sorghum, hybrid, evidenced by vegetati^x vigor 55 

Species of plants, cross-fertilized, effect of selection 36-46 

hybridization and selection 53-54 

self -fertilized, effect of selection 32-36 

hybridization and selection 47-53 

Sports, bud, vegetatively propagated 30-31, 35 

Strawberries, heterozygote species 54 

Surface, Frank M., study of close-fertilized species of plants • 30 

Sweden, Svalof, experiments in plant selection 25, 32 

Swine, crossing, development and causes 69 

Tomato, hybrids, studies 70 

Tomatoes, crossing experiments and results 70- 

northern and southern, alternation, results 61 

study of effect of location 61 

Tracy, S. M., study of cassava , 55 

W. W., sr., study of transformed tomatoes 61 

Tschermak, E. von, study of Chinese oats 70 

hybrid beans 56 

latency 65, 68 

Variations in characters of plants, fluctuating 23-25 

resulting from change of location 61-62 

Varieties of plants, running out 25-26 

105 



7 6 APPLICATION OF PEINCIPLES OF HEREDITY TO BREEDING. 



Page. 

Virginia Agricultural Experiment Station, study of tomatoes 22 

Waid, C. W., experiments in selection of seed potatoes 31-32 

Washington Agricultural Experiment Station, studies in crossing wheat 20 

growing wheat 47, 49 

of hybrid wheat 59 

D. C, studies of dahlia by W. A. Andrews 31 

State, cherries, growing seedlings 54-55 

eastern, wheat growing, new varieties and total acreage 49 

Wheat, beards, development and causes 67 

character, method of determining 52 

close-fertilized species, varieties selected and experiments in growing. . 47-53 

crossing bearded and smooth varieties, result 7, 16, 17 

experiments, Pullman, Wash., results. 20-23 

long-headed and short-headed varieties, results 8, 16, 17 

experiments in selection of self-fertilized species 47-53 

field, experiments in fluctuating variations 23-25 

hybrid, experiments and results 47-53, 59 

hybridization, method of seeming new varieties 49 

reciprocal crosses, experiments 59 

self-fertilized species, effect of selection 32-33 

spring, effect of fall sowing 48 

type, method of determining 52-53 

varieties, careful selection as an aid to maintenance of vigor 26 

winter and spring, effects of crossing 50, 52 

Wilson, E. B., study of chromosomes 11 

Yeast, evolution 29 

Zavitz, C. A., study of mass selection of self-fertilized crops 34, 35 

165 

O 



